This is the updated version of my old Umbral Gloom Monster build.

This build turns [Saryn] into a health tank, while providing a constant massive Gloom slow to a wide area around you (very good for defending objectives), without compromising on the massive amounts of AoE damage she can output. When played correctly, Saryn will be effectively immune to damage until it scales high enough to one-shot her through her health + armor, which is basically impossible on any enemy level in the double digits, and rare until higher triple-digit enemy levels.

There are a couple downsides to the build, though, so I'll get that out of the way first:
1. Very high investment. Goes for 2x umbral forma and a number of expensive archon shards.
2. Is a health tank, meaning you will start to die when enemy levels get high enough to deplete your entire health bar in a single hit. This build is still perfectly suitable for Steel Path, but I've gotten one-shot by a flying corpus rocket laucher guy in an Archon [Hunt] before. Edit: although the latest update just removed enemies' ability to deal critical hits and headshots, so that's stonks for this build! for all I know that rocket was a crit lol
3. Really wants to snapshot conditional strength buffs for both Spores and Gloom, meaning performing a rain dance to stack your Str as high as possible before using your abilities. You know those "one-shot" Elden Ring videos where the guy beats a boss in one hit, but spends the first 30 seconds just applying like 12 different buffs? It's sort of like that. So yeah it's kind of a hassle if you opt to do that.
4. Really wants a good Seismic Bond applicator to free up a mod slot, restricting you to a Sentinel companion (cough dethcube/diriga).

In return for these downsides, you get the most powerful AoE Nuker in the game + a braindead nigh-invincible tank/objective defender all wrapped up in one neat package. It's pretty absurd, actually.

Play Guide

1. Stack as many strength buffs as you can, drop an energy pad if you feel like it, then turn on Gloom. See also Strength Stacking section for more on this. Gloom can be recast at any time, so don't sweat it too much.
2. When you spot an enemy for the first time, once again stack as many strength buffs as you can, and then apply Spores to them. Unlike the Gloom cast you really want to get the strength buff as perfect as you can for this one; remember that the ENTIRE time the spore damage meter is on screen (meaning until you let your spores decay all the way to 0), it is using the strength value you had when you first cast it, so it'll stick with you all mission long if the spawns cooperate. Anyway, assuming the newly spored enemy is anywhere near other enemies, spread those spores by shooting them (while Toxic Lash is active).
3. Continue to spread spores by shooting/attacking already spored enemies (remember to maintain your Toxic Lash buff) and basically spamming the shit out of Miasma. Enemies affected by Miasma and spores will spread those spores when they die, so once enemies start dropping fast enough you will need to make constant use of this to keep the spores alive. Spamming also has the added bonus of increasing your survivability, thanks to the wee augur shield gate and Miasma stun.

Repeat step 3 until the mission is complete. Of course, should the spores drop and start to decay, refer to step 2 instead (no strength stacking needed if spores are merely decaying rather than fully reset). If all enemies around you are spored already, you may proceed to kill enemies with your weapons or do mission objective stuff. If your initial setup step didn't provide enough Str to get you to 272% Gloom slow, be on the lookout for opportunities to refresh it with the slow cap properly applied ([Growing Power], [Power Drain], Molt Augmented, etc). Similarly, should you lose Gloom somehow, remember to buff your Str before reapplying it.

Survivability

3x Umbral + high range/strength Gloom + Augur shield [Regen]; Gloom combined with spore damage will keep you constantly healing at an extremely rapid pace at all times, provided you're doing the thing you're supposed to be doing and spreading spores. The Gloom slow, the shields and stuns from Miasma, and all the raw health/armor takes care of the rest. Try to build your Strength to 272% so you can get a maximum Gloom slow cast off; as mentioned, some combination of Grimoire + Molt Augmented + Molt Vigor + Shards + Growing Power + Hardened Wellspring/Sling Strength + [Power Drain] will get you there. You can also use an arcane slot for additional survivability (details in the Arcane Choices section below). Manifold Arc Coil Diriga with Radiation/Cold can pretty much [Neutralize] enemies too. Oh, and with the recent patch, don't forget you can bust out your archgun and put it away for a quick free +2000 overguard.

Energy Economy

For the most part, we aim for max efficiency + arcane Energize + companion shenanigans for our energy. This allows for fairly effortless Gloom upkeep even under heavy Miasma spam.

New update adds some juicy new options for companions; [Seismic Bond] in particular is a pretty free +30% efficiency if you put it on a beam weapon sentinel (I recommend Verglas). This allows us to crank the build to 11 by freeing up the Streamline slot. Pair with Energy Generator or Mystic/[Manifold Bond] [Diriga] for even more energy. (If not using Gloom, I'd also recommend [Archon Stretch], as companions can proc that too. But alas, Gloom.) [Mystic Bond] can slightly decrease survivability tho, since 0 cost skills don't regenerate any shield.

Stat Explanation

Unfortunately, [Saryn] is one of those rare frames who values all 4 stats. Strength is very important for both damage output and Gloom slow %, Range is paramount to her spore spreading mechanic, Efficiency is key to a sustainable energy economy on a caster+Gloom combo, and Duration is needed for Gloom energy drain as well as uptime on Toxic Lash buff/Miasma debuff.

Efficiency I just talked about at length; suffice to say as an avid [Saryn] player I've tried a bunch of stuff and in the end I'm happiest sitting at 175% efficiency, especially on a Gloom build. (A build more suitable for level-cap would want less efficiency, due to how the 'shield gate abuse' survivability playstyle requires more expensive abilities (plus decaying dragon key) for full shield refreshes, but that's a totally different build.)

Range is second fiddle only to efficiency in my book when it comes to modding; sources of Range are quite rare, so you have to make sure your mods provide enough. [Saryn]'s whole deal is getting her spores spread and keeping them as spread as possible, and her ability to do that scales directly off Range.

Strength is also extremely important, so if you haven't been reading closely you might be confused at my leaving it at 117%. That's because this is where you dump everything that's not a baseline mod into Strength. See also Strength Stacking section.

and finally, Duration is kept above 100% by keeping [Fleeting Expertise] deranked by 1, and compensating with [Primed Continuity]. That's all that's needed, just to keep the Gloom drain at max efficiency and keep Toxin Lash from having an annoyingly short uptime. Also dictates Miasma sickness duration (base 6s) and spore decay rate. More would be nice, but 105% is enough, really. (Note that [Archon Continuity] only procs on Molt from [Saryn]'s kit, which is replaced by Gloom in this build anyway, so it's strictly worse than Primed.)

Strength Stacking

Here's a list of ways to get more Str in game:

• Madurai Sling Strength (+40%)
• Molt Augmented (up to +60%)
• Molt Vigor (+45%)
• Power Drain (+50%)
• Growing Power (+25%)
• Grimore (up to +60%)

and not used in this build:

• Energy Conversion (+50%)
• Zenurik Hardened Wellspring (+20%; incompatible with Madurai so not recommended)
• [Nidus] Specter (parasitic link. that's cheating yo)

I usually don't use Augmented + [Vigor] together since I find Energize is too good to pass up (the more you spam Miasma, the higher your KPS will be). Jury's out on whether Augmented or Vigor is better; Augmented is basically +60% to Miasma/Toxic Lash whereas [Vigor] is +45% to Spores. [Vigor] will also let you cap Gloom slow right at the start of the mission, too. Though Augmented also makes it cost less to manually re-spore which you have to do occasionally, so that's another point in its favor. Personally I've just found Vigor to work better overall, as long as you try not to re-spore too much.

Anyway yeah, just cast 2 + double-sling to prep your Sling Strength/Vigor buffs and hit a stack of enemies with your Grimoire's alt-fire until it hits +60% and tada you're all buffed out. Only thing to add is an Eximus mercy kill for +50%, and of course Augmented, but you don't really have to think about Augmented, thankfully.

Arcane Choices

There are basically 3 main choices: Molt Augmented, Molt [Vigor], and Arcane Energize. [Saryn] loves strength, and needs energy. Other choices simply do not hold much of a candle in comparison.

Arcane Energize is pretty much a must-have for me, you will fall behind in kills if you can't spam the hell out of Miasma which you need some heavy duty energy [Regen] option for. It's basically either this, [Equilibrium], or Nourish, and this is a Gloom build that can't spare the mod slot, so Energize it is. In truth, it might ultimately be optimal to drop some umbral health tanking mods in favor of [Equilibrium] + double Molt arcanes given how little you actually get hit under Gloom, but that remains to be seen.
Molt Augmented seems like even more of an auto-include than it usually is, given [Saryn] loves strength and can rack up a massive kill count in no time. However, its lack of presence at the start of the game means your Spores won't be able to take advantage of this Str, and you may have to settle for a non-capped Gloom slow initially.
Molt Vigor fixes that problem by giving an up-front on-demand +45%. However this demand is too great for regular spam usage, so this largely will not apply to Miasma, and of course in comparison to Augmented it's 15% less strength total. Tough choice.

and to discuss a few other arcanes that feel worth mentioning, at least:

Arcane Guardian: Solid extra armor if you still feel a tad too squishy, but it has anti-synergy with Gloom (you get hit less, making it proc less often) and it isn't too impactful since you already have 3x Umbral granting you plenty of armor.
Arcane Tanker: [Guardian] but better and more reliable, at the cost of being an absolute pain in the ass instead of 'set and forget.' Not worth in my book, but if you wanna hit a comfy 2k armor reliably here you go.
Arcane Blessing: Gives +1200 health at max rank, another way of getting extra defense. Sadly the recent frame stat update made it so 3x umbral [Saryn] no longer sits at 1337 health, so this is now a viable choice.
Arcane Double Back: Yet another defensive arcane, by far the most effective of them all if you can keep it stacked (75% DR is much better than a little over double health/armor). However keeping it stacked means spamming parkour maneuvers constantly so I'm not a huge fan (also makes knockdowns much more risky as you stand to lose your stacks).
Arcane Aegis: Final defensive arcane to consider, effectively gives you 12s invincibility against anything that doesn't deal direct health damage when it procs...however the proc is a 3% chance on shield damage, which when combined with a big Gloom slow is very unlikely to trigger with regularity. So honestly, I wouldn't use this, just adding it for comparison's sake.
Arcane Consequence: I mean, +60% parkour velocity for 18s that's pretty easy to trigger. If you wanna shmoove it's not a bad pick.
Arcane Agility: Same as consequence but with a different trigger condition. 60% chance on being damaged rather than 100% on landing a headshot. Seems worse to me, especially given Gloom slow, but up to you.
Arcane Steadfast: This is just a bad Energize, don't pick.
Molt Efficiency: Often basically just +36% duration, but doesn't have anywhere near 100% uptime and Duration is our least cared about stat. Rather underwhelming IMO.

Other Mod Explanation

You also might notice Revealing Spores over Primed Sure Footed. I know, not running PSF on a frame that has 0 forms of natural knockback resistance/immunity? Yikes. But Saryn doesn't care much for explosive weaponry, so the self-stagger concern isn't high, and while enemies can knock her around, this doesn't really matter when your spores are still working at [Full Capacity]. You'll still be doing plenty of damage and healing rapidly, so think of it as a quick break. Also, a recent update buffed archgun deployment to give you +2000 overguard, which stays after putting the gun back away and can function as a replacement for PSF while active; this is surprisingly effective given how little you wind up getting shot when all enemies near you are either 95% slowed or dead.

Meanwhile, I've come to fall in love with [Revealing Spores]. It's so good! For one, I already really like [Enemy Radar] and generally find it to be an undervalued stat, and a cool +40 is real nice to have. Lets me run [Growing Power] instead of [Enemy Radar] and not feel like I'm nerfing my radar. But even more importantly, the ability to see at a glance exactly which enemies are infected and which are still unaffected is invaluable. [Saryn]'s whole playstyle revolves around getting her spores spread as effectively as possible, and this mod makes it as easy as trying to turn the red minimap icons into green ones. It's also just inherently really satisfying to watch all those red enemy icons light up green. [Saryn] is a gardener!

The downside is I could be getting another +15% range here, but once you go [Revealing Spores], you never go back.

Archon Shards

The name of the game used to be Strength shards, but now it is emerald shards. You want 2 normal greens for +4 corrosive stacks to allow spores to full strip armor, and ideally 2 tauforged greens for 1.3x ability damage to corroded enemies. After that I slot 1 tauforged yellow for cast speed comfort, but this is a flex slot on a health tank build. Throw on yet another tau green for 1.45x dmg if you really want to make a point of it.

Conclusion

I might argue Saryn is overpowered as shit. In the past, she used to always be that frame that was best-in-class at AoE nuking every living thing on the map, but the tradeoff was that she is squishy, and her energy economy needs would fight against her survivability/offense needs to the point it was pretty tricky to build and play her, and she wasn't a great pick into a lot of kinds of content. No longer. Now she remains best-in-class AoE nuking every living thing on the map (and by a lot, it's not even close), but now she can also simultaneously be a braindead tank that excels at defending objectives. Gloom and all the new free Strength (archon shards + Molt Augmented) broke her. Good stats + Gloom = nigh unkillable murder machine who barely even has to play the game. Seriously, spore spreading is not that hard, especially when you have [Revealing Spores] as a visual aid. Just get those minimap icons green and take a nap, marvel as you outpace all your teammates in kills by a huge margin while borderline AFKing. Saryn is nuts.

Changelog

1/14/24: New patches brought enough new changes it was time for an update. [Seismic Bond] gives a free +30% efficiency, freeing up a mod slot to finally make room for [Augur Reach]. Emerald shards give us full armor strip on spores and 1.3x ability damage, both massive buffs to [Saryn]'s damage output. This requires 4 shard slots, meaning a loss of up to 60% strength...except the devs also gave us the [Grimoire], allowing us to get it right back. So basically the build now benefits from +1 Augur Reach and those massive emerald buffs, with the only cost being some finicky-ness thanks to needing to stack the Grimoure buff and [Seismic Bond] being somewhat less reliable than [Streamline]. Very worth in my book. Reached over 200 kpm in solo SP Circulus testing out the changes (over 2k kills before 10 minute mark), while very easily surviving thanks to massive Gloom slow over a massive range. gg ez