ALL MY BUILDS FOCUS ON USING UNDEAD-THEMED WARFRAMES WITH WEAPONS FITTING THEIR THEMES. THEY MAY NOT BE THE ABSOLUTE BEST OPTION FOR IT, BUT I STILL MAKE SURE THEY'RE MORE THAN VIABLE ENOUGH.

[Revenant Prime] represents the Revenant.

INTRODUCTION

The main focus of this build is to be able to be able to solo content and provide the most support in Index, Netracells, and Void Cascade, while also being able to lazily do Profit-Taker. Considering these game modes all play differently I have 3 different setups for them, and will show them below while specifying why I changed certain mods. Index and Profit-Taker use Eclipse as an ability, while the Netracells/Void Cascade variant uses Roar.

INDEX

As for our changes... Index has 2 main mechanics. First is that we don't get to use our companion, and second is that we have to collect index points from enemies, and the more points we collect the faster our energy will drain. With that in mind we replaced the aura to [Enemy Radar] because energy regeneration is useless, and it'll be better to be able to find the enemies from a greater range to collect points.
[Stretch] and [Mesmer Shield] are replaced with [Fleeting Expertise] and [Streamline] to be able to cast Mesmer Skin even at max index drain.
Arcane Steadfast is replaced with Molt Efficiency to keep Eclipse up for as long as we can now that we have a lowered duration.
That said, here's the steps to follow for a successful Index run:

0. Make sure you invert Mirage's ability in Options -> Invert Tap/Hold Abilities (Do it on both keyboard/mouse and controller settings). That way you'll only have to Tap Eclipse to get the damage buff.

1. Go towards the energy drop area and do chained slings as your operator before getting it and casting Mesmer Skin, then do the same to cast Eclipse.

2. Kill enemies and collect points. The max bonus you get from points is +10 with 20 points, but the optimal is +8 with 15. If playing for long rounds this setup lets us be the designated point collector as well, letting others bank point while we collect as many as possible to bank them all next round.

3. From time to time repeat step 1. Power Transfer gives you faster cast speed when you switch in and out of operator, so taking the extra step to trigger Sling Strength is not as bad as one might think.

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PROFIT-TAKER

As for our changes... First is that we won't entirely focus on killing enemies for anything other than a few stacks for our Zaw, and second is that we want to replace our companion with a beast so we can slot it either Prosperous Retriever if credit farming (in which case a [Chroma Prime] might be a better frame to use) or Resourceful Retriever if farming Crisma Toroids for Little Duck, which is what we do.
With that in mind we replaced [Stretch] and [Mesmer Shield] with [Fleeting Expertise] and [Energy Nexus] to have just enough energy regeneration to cast both Mesmer Skin and Eclipse whenever we need to.
Molt Augmented is replaced with Molt Vigor for a burst in ability strength since we won't be killing enough enemies for the former to matter.
Arcane Steadfast is replaced with Molt Efficiency to keep Eclipse up for as long as we can now that we have a lowered duration.
That said, here's the steps to follow for a successful Profit-Taker run:

0. Make sure you invert Mirage's ability in Options -> Invert Tap/Hold Abilities (Do it on both keyboard/mouse and controller settings). That way you'll only have to Tap Eclipse to get the damage buff.

1. Summon your Itzal to fly to the Profit-Taker location. When you get close enough to the Profit-Taker Orb go into operator, cast Contamination Wave and do chained slings, then cast Mesmer Skin and Eclipse.

2. Hit the Profit Taker with the element required and make sure to double jump and do a contagion zaw throw to clear out enemies and build galvanized charges on our Zaw. With my build you should only miss toxin , so whenever that comes out shoot the Profit-Taker with your amp. You can also use a Pharaoh Predasite to get the extra toxin damage, use an on-call crew with a weapon modded for toxin, or mod your archgun for toxin on your Necramech.

3. When the shields are down summon your archgun and AIM before shooting the legs with it, then bring down its health, unsummon your archgun and [Rush] to the pylons to break them.

4. Repeat the process until it dies, then cast Cosmic Crush with your [Itzal] to get the drops before quickly going back to Fortuna.

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NETRACELLS / VOID CASCADE

As for our choices here... We use Roar instead of Eclipse because it can buff our allies and allows us to one-shot with Reave.
[Stretch] fixes the range of Roar so taht we don't have to stand next to our allies to buff them, while also making Reave's radius usable. [Mesmer Shield] gives us more stacks of Mesmer Skin and allows us to give 5 stacks to our allies within affinity range, keeping our party immortal.
Arcane Steadfast and our companion both give us free ability casts, so we don't really have to worry about efficiency.
That said, here's the steps to follow for a successful Netracells run:

1. Take all keyglyphs so your party will be unburdened, then switch to operator, do chained slings, and cast Mesmer Skin while your allies are in range

2. [Rush] to the terminals, find the vault, then go to the marked circle area and kill enemies while staying inside the circle. recast Mesmer Skin whenever you or your allies need some stacks, and keep Roar up as well. Whenever a Necramite spawns you can [Rush] to kit and kill it before continuing.

And for Void cascades:

1. Switch to operator, do chained slings, and cast Mesmer Skin to [Protect] yourself and allies. Then do the mission as normal, closing tears, killing thrax, and protecting the exolizers.

2. As the mission goes on you can use Enthrall and Reave to crowd control and one-shot enemies so you can mostly focus on the thrax around. If doing a public run and your companions need protection then make sure to replenish their stacks often with Mesmer Skin.

BUILD SPECIFICS

FOCUS SCHOOL

We use Madurai for this build. The nodes we care most about are:

• Phoenix Talons: This gives us a slight damage boost. Everything helps so we might as well take it.
• Power Transfer: This gives +50% casting speed, freeing up a mod slot.
• Void Strike: This gives us +1000% damage to our amp and weapons for 8 seconds, making things trivial if you're fast enough. Sadly it has a 40 second cooldown its best use is giving extra damage for when you haven't built up galvanized stacks, or for your weakest elements on the Profit-Taker fight.
• Void Fuel: This gives us a 40% chance to not consume ammo while Void Strike is active, which helps quite a bit for sustained damage.
• Sling Strength: This gives us +40% ability strength for 20 seconds after consecutive chained void slings. Perfect for buffing Mesmer Skin and Roar/Eclipse. we're already switching to operator from time to time for cast speed, so might as well add ability strength to that.

ARCHON SHARDS

• Crimson Archon Shard:
  +15% Ability Strength x5. Strength is what you want most for [Revenant Prime]. We alredy have a decent duration for Entrall if we want to use it, and you won't really use Reave in the Index or PT fights, but it's still great for regular use at higher levels. What you really want is more Mesmer Skin charges. Mesmer Skin counts decimals as extra charges (IE 20.001 charges are actually 21 charges, with the last one appearing as a 0). I'll be counting charges like that.
  By default we have 293% ability strength, which is 368% with all our shards. On top of that we can add +40% with Sling Strength and up to +60% with Molt Augmented. That'd make the values be:
  By Default:                                            368% ability strength for 23 Mesmer Skin charges and a 137.4% buff from Roar.
  With Sling Strength:                           408% ability strength for 25 Mesmer Skin charges and a 149.4% buff from Roar.
  With max Molt Augmented stacks: 428% ability strength for 27 Mesmer Skin charges and a 155.4% buff from Roar.
  With Both:                                             468% ability strength for 29 Mesmer Skin charges and a 167.4% buff from Roar.

STATS:

• STRENGTH: We use [Power Drift], [Blind Rage], [Precision Intensify], [Transient Fortitude], [Augur Secrets], Molt Augmented (or Molt Vigor for the Profit-Taker fight), and all archon shards. What we care about most on the Index is killing enemies fast and ignoring the index point debuffs. With a minimum of 368% ability strength total we'll definitely get more than enough to fulfill both conditions. Strength is the most important thing for [Revenant Prime], so you wanna stack as many ability strength mods as possible as long as they don't give huge disadvantages.
  The other ability we care about that relies on ability strength is Roar/Eclipse. The bigger the buff we get the better, of course.
  Speaking of roar, if we ever wanted to one-shot enemies with it and Reave we can with our current build, as long as we have max Molt Augmented stacks. The proper calculations work like this:
  Our base ability strength is 293%, then +75% from our archon shards, +40% from Sling Strength, and +60% from Molt Augmented for a total of 468% ability strength.
  Reave drains 8% of an enemy's max health at base, so 8% x 468% = 37.44% of their max health.
  Roar uses [Precision Intensify] for +90% on top of the 468% ability strength that we have, for a total multiplier of 30% x (468% + 90%) = 167.4%
  This works for ability damage, so it's added on top of Reave's new value. 37.44% x (1 + 167.4%) = 100.11% which is effectively a one-shot, and the only thing we have to worry about is having Sling Strength active before casting Roar and using Reave.

• DURATION: We use [Narrow Minded] (and Molt Efficiency for Index and Profit-Taker). By default Narrow Minded is more than enough for us, but since the Index and Profit-Taker sets use [Fleeting Expertise] we have to make up for it by using Molt Efficiency then. This arcane will be fully active most of the time since Mesmer Skin also protects our shields, but certain traps do go through it. Thanks to this we'll end up with 147.5% ability duration, making Eclipse last 36.9 seconds.
  For regular use at 171.5% ability duration Roar lasts for 51.5 seconds, which is more than enough.

• RANGE: We don't use any range mods for Index or Profit-Taker, but we use [Stretch] otherwise. Eclipse needs an augment to buff allies, and with how much they move in the Index it would be worthless to slot it in. Profit-Taker is basically done solo, and neither of those game modes require Reave so range is useless. For Regular use and Netracells we use [Stretch] to make up for the loss in range, that way we get a 19.75 meter radius buff on Roar to buff our allies with, and Reave becomes usable again.

• EFFICIENCY/ENERGY: We use Fleeting Expertise and either Streamline for Index or Energy Nexus for Profit-Taker. For regular missions and Netracells we use Arcane Steadfast. Why not [Primed Flow], you ask? Well, as stated in their respective sections, we do have our energy drained when we have enough index points collected at once, so we would almost always be at 0 energy. Index provides energy orbs, however, and they give 50 energy each, so we use efficiency mods to lower the cost of Mesmer Skin enough so that we can cast them immediately after collecting the orb even at max energy drain. This is the absolute minimum to make it possible to cast Mesmer Skin with Index energy orbs at max Index debuff, as long as you spam the ability button as you collect the orb.
  As for the Profit-Taker, [Energy Nexus] should be more than enough considering we only cast Eclipse once every 40 seconds or so and Mesmer Skin every 25 seconds or so.
  With regular use we have Arcane Steadfast and our companion providing us with free ability casts while the latter also gives some energy orbs.

LOADOUT:

• PRIMARY: We use the Felarx. This weapon is extremely powerful, able to 1-shot most enemies with the right setup. Thanks to the side effect of Mesmer Skin making enemies fall asleep it's easy to get headshots as well, so you should be able to have the arcane stacks up most of the time.
  I chose this weapon thematically because revenants are vengeful spirits that hunt people to enact revenge, and this is a hunting gun with supernatural properties.

• SECONDARY: We use the Kuva Nukor. This weapon is perfect for priming enemies, but it's also able to deal pretty nasty damage on its own. It also provides a large amount of the elements required for the Profit-Taker fight.
  I chose this weapon thematically because the beam looks like a stream of ectoplasm when coloured correctly.

• MELEE: We use the Ooltha. This gives us the last elements we're missing for the Profit-Taker fight, while still offering decent damage overall. It's modded for gas and electric, but Exodia Contagion also adds blast. It technically adds viral as well, but that element is too finnicky and only deals damage if you hit the Profit-Taker from behind.
  I chose this weapon because in most media revenants tend to be vengeful undead knights wielding longswords. The Longsword Frysta Skin seems to work best for a simple yet elegant look if coloured correctly.

• COMPANION: We use Dethcube Prime. It doesn't really matter what you choose for Index because you can't use companions there. For Profit-Taker the only thing that matters is that you choose a living companion instead of a robotic so you can use Prosperous Retriever/Resourceful Retriever . For regular use and Netracells [Dethcube Prime] is a much better choice, using Mystic Bond for an extra free ability cast while Energy Generator provides us with energy orbs fairly regularly.

• ARCHWING: We use Itzal. Cosmic Crush is the most important thing about this archwing, letting you get the Profit-Taker's drops without worry.

• ARCHGUN: We use the Mausolon. It's the best archgun for general damage, while also being one of the best for Profit-Taker. Modded for magnetic to quickly melt through the Profit-Taker's limbs. Make sure you're using this build on the "Heavy Weapon" section of your arsenal, since we're using our archgun deployer to take advantage of Roar, and also make sure to aim before shooting for the extra crit chance and damage.

PROFIT-TAKER ELEMENTS:

With our current setup we end up dealing all elements besides Toxin with decent damage on each. This is how they're distributed:

• Impact /Puncture /Slash : Felarx

• Heat : Kuva Nukor
• Cold : Felarx
• Electricity : Ooltha
• Toxin : None. The main options for it are a Pharaoh Predasite, an On-Call crew with a weapon modded for toxin, or a Necramech with an archgun modded for toxin.

• Blast : Ooltha
• Corrosive : Felarx
• Gas : Ooltha
• Magnetic : Felarx or Kuva Nukor
• Radiation : Kuva Nukor
• Viral : Kuva Nukor

GAMEPLAY ELEMENTS:

• SHIELD GATING: Even though we use [Augur Secrets], we don't really need any kind of protection for [Revenant Prime]. Mesmer Skin gives [Revenant Prime] a certain amount of charges based on ability strength. Every time we get hit, we get the damage reflected, stun the enemy that hit us, and lose a charge instead of taking damage and become invincible for a second. While invincible we won't lose any charges, so this effectively prevents us from dying. Of course, whenever we re-cast Mesmer Skin we'll be vulnerable, but the regular invulnerability from shield gating should be enough for our casting animation.

• AUGMENT: You could consider [Precision Intensify] as an augment for Roar/Eclipse, since it gives us +90% ability strength at all times on that ability.
  For regular use we also have [Mesmer Shield], which provides our allies with 5 stacks of Mesmer Skin and gives us +50% ability strength on that ability.

HELMINTH OPTIONS

For the buff option we have 2 abilities we might want: Roar and Eclipse.

• Roar: Roar's buff is pretty consistent. It works as a faction buff, so it's most effective when you don't have faction mods and do status effect DOT. It doesn't double dip on attrition evolutions, so it's also more useful when not using Incarnon weapons. It's also an overall buff to damaging abilities like Reave. It can also be shared with teammates.

• Eclipse: Eclipse is way more complicated to explain, but I'll try. Basically, it doesn't buff abilities or work as a faction modifier, but instead is its own separate modifier that multiplies damage. This works differently with things like galvanized mods and [Condition Overload]. Since the galvanized mods that add extra damage per status work the same as [Condition Overload] I'll just call them all CO. To keep things simple (since the cases vary from weapon to weapon):
  If a weapon has additive CO (For example most hitscan weapons) then CO just adds on to the base damage, ignoring all other modifiers. If a weapon deals 100 damage, has a 525% damage multiplier and 122.4% from Eclipse that's 100 x (1 + 525%) x (1 + 122.4%) = 1390% damage with eclipse, adding 3 status effects will just increase it by 120% for 1390% + (100 x (40% x 3)) = 1510%. Not that much and has diminishing returns the more of either multiplier you have.
  If a weapon has multiplicative CO (For example most projectile weapons that have travel time and aren't AoE explosions) then CO multiplies on top of that. with the same example you get 100 * (1 + 525%) x (1 + 122.4%) x (1 + (40% x 3)) = 3058% damage. You can see the difference.
  And as a last point, it works multiplicative TWICE on thrown melee and contagion zaws.

So as we can see, there's a few of situations to choose from, but I'll try to summarise them to help you choose.
Are you planning on buffing allies or Reave? Then Roar, no contest. Eclipse needs an agument and to stay close to your teammates for the former, and can't do the latter.
Are you dealing most of your damage through status DOTs? then Roar, since it works as a faction bonus and double dips on status damage.

Are you doing raw damage and using CO mods? Then:
Does your weapon have additive CO? (Hitscan weapons and others). Then Roar and Eclipse are functionally the same, and make CO kind of useless.
Does your weapon have multiplicative CO? (Projectiles with travel time like [Felarx]). Then Eclipse provides a better buff since it's multiplicative.
Do you use a Contagion Zaw? Then Eclipse is the clear winner, applying multiplicatively Twice on direct hits.

BONUS BUILD - 0 FORMA OPTION

So, if you want to test this build but are too worried that you won't like it and will waste forma on it, here's a 0 forma version of the build for you to test. 0 Forma builds always have to make some compromises, but I try to make them as close to the original to give an accurrate representation when testing.

MOD CHANGES:

• [Power Drift] removed: We just cannot, in any way, shape, or form, add this mod with the amount of polarities we have. For Index and Profit-Taker we add Mobilize just as a QoL mod, but it's unnecessary.

• Augur Secrets removed (On Index and Profit-Taker sets): We still don't have enough polarities with how much other mods cost, so we removed teh smallest ability strength increase we have.

And that's it! Besides removing the exilus and one other mod in 2 sets the mods are exactly the same, just in different spots since we can't change polarity positions with 0 forma. You'll only lose on a bit of strength, but overall it has the same potential. The only main difference is that you won't be able to one-shot with Reave.