Post plague boi prime drop update: actual prime build here.
Planning out [Nidus Prime] for next week. Won't know actual forma count until then, too, but an Umbral is a safe bet.
Arcane Nullifier because I absolutely loathe getting hit by sneaky magnetic procs, let alone on a frame/build with an emphasis on spamming a power; it's replaceable with whatever you prefer. Grace for more survivability is a good choice. Nidus (and by extension, Inaros) gets less out of Energize since Hunter Adrenaline/[Rage] procs every time you take a hit, but is still a perfectly fine choice as well.
Many players struggle to gain stacks quickly, to the extent that some can't imagine the [Abundant Mutation] augment as actually being useful. After all, if one is having trouble reaching 50 stacks, what good is an augment that increases the cap from 100 to 300? The trick to it is simply to use all four abilities in-tandem. Do not wait for the perfect chance to cast Parasitic Link and Ravenous. Once you have a stack, hit an enemy with Parasitic Link the next time you see a group, cast Larva to group them together, and spam Virulence at them until they're dead. Try to grab as many enemies as possible by casting Larva reletively close to the enemy you're tethered to with Parasitic Link, so the copy of Virulance that gets cast from their position whenever you cast it yourself will also hit the Larva ball of tangled enemies, thus doubling your stacks. Depending on enemy level, unless they're so low that Virulence is a one-hit kill with no stacks, you should gain quite a few stacks from this first group alone. Repeat this with the next group, but cast Ravenous after linking to an enemy, and cast Larva over Ravenous to draw the group of enemies to where [Nidus]' puppies are spawning so they'll immediately get jumped by them. If the enemies are tanky (basically guaranteed if you're on Steel Path,) you can cast Ravenous right where Larva has your very first group balled up once you've gained those starting stacks back. If the enemies are hardy enough, once they're dead, the stack counter will have to actually catch up because it displays its increments at a set animation speed. You can easily get 30-50 stacks at the start of an Exterminate mission in less than 20 seconds from when you find an enemy group to exploit (again, enemy level is a factor here.) It gets far more ridiculous in an endless mode where enemies will swarm you more reliably and even more if you're solo'ing; it's quite possible to [Reach] 100 stacks without realizing it because the counter takes that long to catch up.
Here's the thing, though. [Nidus] has an important mechanic that's easy to miss, and it very likely contributes to the way players struggle to get a grip on just how they're supposed to make this frame work. If you've never looked up [Nidus]' mechanics on the wiki or read the tips on his ability screen, prepare to have your mind blown. The mechanic is simple; Virulence does more damage for every mutation stack Nidus has. Mutation stacks are a modulation mechanic, not a race to the cap. If your Virulence is one-shotting Eximus units, the problem isn't that it's too hard to gain stacks, the problem is that you already have enough. You don't need anymore than what you have plus another 15 just in case Undying needs to be triggered. (Sidenote; even at 300 stacks, Virulance can't one-shot enemies once they're beefy enough, and it's basically never going to happen in Steel Path. This does not make Virulence useless for combat at high level, however; it will still take big chunks of health off most enemies and fighting with it will still be completely viable. When you're fighting enemies that beefy, you'll just keep spamming Virulence into groups yanked together by Larva to kill them instead of for more stacks. It's also still worthwhile to keep Parasitic Link on an enemy to double the damage Virulence does and to maintain your high damage mitigation.)
This build is missing out on duration because Parasitic Link and Ravenous already have good duration as it is (60 and 40 seconds respectively,) while leaving Larva at it's base duration obviates the need for Larval Burst. Now that we know how to gain stacks like a boss, it means we don't have to worry about having the stacks on hand to re-new these abilities when their base duration expires. If you want to avoid downtime with Ravenous, stand on it (so it doesn't spawn a new one and reset the puppies) and cast with two seconds left on the timer.
Gaining stacks like this makes the [Abundant Mutation] augment far easier to take advantage of (if you're using it, remember to run away when you see Undying proc; get away from the group of enemies who killed you so they can't kill you again while Undying is on cooldown. Get yourself into a less hectic situation to replenish the spent stacks, or just keep retreating until the cooldown is done to play it safe.) A good variant is to replace [Flow] with [Abundant Mutation] for solo play. Understand, however, that the main benefit of this augment is not giving you extra chances to trigger Undying without having to refill your stacks in-between; the fact that the augment nerfs Undying by adding the 30 second cooldown completely nullifies this as an advantage. The real point of this augment is to further buff Virulence so very high level and/or Steel Path enemies will die to fewer casts because its mutation stack damage buff can get higher than 100.
Gaining stacks like this also makes the [Insatiable] augment redundant; when you're gaining stacks so fast you don't even realize you've reached 100, you just don't need it. If you're going to run it, it should be alongside [Abundant Mutation] to [Reach] 300 faster, but even then, I think it's a waste of a mod slot. The simple fact of the matter is that if you're struggling to gain stacks in the first place, this augment is not the solution, the solution is to simply adjust your playstyle so you'll get more out of [Nidus]' powers.
We're stacking at least two Carapace mods (you can drop [Flow] or [Stretch] for the third, although at that point I'd rather keep the extra utility than gain another 2000 effective HP) because they're more bang for the buck than [Adaptation]. Because of the way [Adaptation] works, you'll never have true 90% resistance against all incoming damage. With the full Umbral set, using Parasitic Link on an enemy already gives you over 90% damage reduction without having to stack a buff (and Parasitic Link also isn't taken into account for the 16k effective HP given here with conditionals applied.) If [Nidus Prime] has extra armor, it probably won't be enough to really notice at this point, but hey, more is more.
If your personal preference is different and you want maximum tank nonsense, you can, of course, drop both [Flow] and [Stretch] for the third Carapace mod and [Adaptation], because while [Adaptation] is more complicated than it seems, it's still good, and stacking it along with the rest will increase survivability even further despite it not as much as one might think. If you do this, leave [Adaptation] unranked to save yourself a Forma; ranking up [Adaptation] does not increase the maximum protection it gives, only the amount each proc gives until the maximum is reached.
You can, of course, go the other way and drop the Carapace mods for other mods you like, presumably augments. [Nidus] will still be very tanky, and if you're gaining stacks efficiently, [Undying Will] save you anyway.
Aura can be whatever you prefer, so [Corrosive Projection] is, of course, a good choice. [Physique] seems like the obvious choice, but it's crap since it only applies to base health, giving [Nidus] a whopping 405 extra HP. Not a bad chance to use [Shepherd] to give pets a longer life since your Aura mod doesn't need to help Nidus survive at all.
Flow is less important when solo'ing since you'll be shot (thus proccing Hunter Adreniline/[Rage]) far more reliably, although for my money I still prefer to have it to increase the chance I'll be able to spam Virulence willy-nilly any time I want. You can also drop [Flow] and just stay away from your squaddies when doing an endless mode. Have a good melee weapon as a fallback for when you run out of energy for Virulance while Larva is still active, so you can fall back on it to quickly cut down the balled up enemies.
A major problem is that alternative mods like the augments or mods for alternating between things like range, durability or power strength don't have matching polarities with what they're replacing, and a full three Umbral formas won't allow the necessary wiggle room for at least some of them (I think, I haven't tried everything personally and on top of that I have a hard time keeping things like this straight in in my head) which means either settling on exactly what you want or building a second [Nidus]. The third alternative is to simply not do a full Umbral build. For me, that falls into "why bother" territory. [Nidus] is a tank with no shield, the entire idea is to jack his health and armor up as much as possible. Plus, not needing shield mods means you can use the full set while still leaving you spoiled for choice on what to put in other slots. Plus again, you'll probably have to drop the entire Umbral set, not just Intensify, which makes it worse.
Nidus is garbo at one thing, and that's solo-protecting an excavator on high or even mid-level excavation. At the very least, you can't sleep through it like many frames can, there are frames that can't sleep through it but are still better than [Nidus], and if you have [Nidus] you probably have at least one frame in either of those categories. In other words, solo Excavation with [Nidus] is doable but you're making it so much harder on yourself you may as well switch frames anyway. Not soloing means you're basically supporting the squad with heals and power strength buffs by linking to them.
We'll go over howe2nidus on Excavation for completeness' sake; Larva will often miss enough enemies for the excavator to be destroyed, so you have to watch for them. If you swap something out for Larval Burst, you can spam Larva to keep catching enemies; it's kind-of clunky, but it helps immensely. Also get Ravenous up ASAP so the puppies will distract incoming enemies/enemies not caught by the hentai ball as well. Having a weapon that can efficiently take out groups is basically necessary for this, especially in Steel Path, like a beam weapon (Amprex, Ignis) to sweep around or anything that explodes (Lenz, Zarr, Bramma, Ogris, etc) or plan ahead to use your arch-gun (probably an [Imperator] or [Mausolon], preferably [Mausolon] to mow enemies down. I suspect the [Fluctus] will drop off in effectiveness too fast at higher levels.) You lose the chance to gain stacks by making enemies go all explodey, but your priority is protecting the Excavator. Essentially, this is a situation that [Nidus] just can't handle with his powers alone, which is not uncommon, but it's very jarring with [Nidus] because he can handle every other situation while pretending his weapons don't exist.
In my opinion, and I'm going to stress in my opinion on this one because your playstyle may vary, nothing is really worth it from Helminth for [Nidus], it's just too important to use all four of his abilities in tandem with each other. You obviously can't replace Virulence, since it's the only way to gain stacks. Replacing any of the remaining three will seriously nerf your stack gaining. Beyond that, replacing Ravenous would probably be the safest bet, since the extra healing from standing in it is probably the least important part of making Nidus super-durable, but you lose out on [Nidus]' puppies greatly increasing your stack building by using Larva to group enemies right on it. Replacing Larva also seriously nerfs the speed you'll gain stacks at and replacing Parasitic Link seriously nerfs your survivability, once again nerfs the speed you can gain stacks at. Even if you're okay with losing out on stack-gains and whatever else one of those abilities gives, there's really nothing worthwhile to replace them with. Because of the way he works, Nidus doesn't need a non-native ability to increase his ability strength. Nothing that adds more tank durability like armor or healing or whatever will add enough of it to justify losing out on stack-gaining; [Nidus] is already an insane tank and if you're gaining stacks as fast as I've described, you'll have more than enough for Undying in a long endless mode run, so it really just makes Nidus less efficient. Nidus is not a frame for whom the wheel needs to be re-invented.
As a point of order, the crowd-control of Larva makes [Nidus] an excellent choice for using Helminth for other purposes. As an easy example, Grind Hard Squad has a guide here where he replaces Ravenous with Sickening Pulse, then uses Larva to inflict ludicrous Electricity damage on huge groups of enemies. As I said above, to me it's still not really worth it, I'd rather just use Helminth to put Larva on a frame that doesn't depend as much on ability synergy. Of course, you lose out being able to pair Larva with a second Helminth'd ability like Sickening Pulse, but again, this just isn't my thing. I like my Nidus at maximum stack-gaining speed. Your mileage may vary.