(skip to 'Flex Slot' part for explanation of what you should put in the empty slot)
Evo II: Final Fusillade (Increase Damage by +51. +3 Multishot on last shot in magazine.) We take this because the other perk (Plentiful Mayhem) is absolute dogsh*t and actively makes the weapon worse.
Evo III: User preference. I prefer +9 magazine size as it allows you to freely spam shots to get back to incarnon mode quickly, but +50% projectile velocity is good too. The other one seems pretty meh, ideally you never even reload with this weapon.
Evo IV: Commodore's Fortune (+20% Crit Chance). We completely neglect status chance on this build because this is a pure viral weapon; viral caps at 10 stacks, and this weapon reaches that cap very quickly even without any bonus status chance.
Fire rate is really, really good; greatly scales the damage output and it's so easy to get back to incarnon there really aren't any ammo concerns. IMO Vile Acceleration is a must-have if you're not getting more fire rate from another source. Note that punchthrough has no effect on the beam weapon (and is actively detrimental to the base weapon), so don't use any [Shred] variant.
[Primed Firestorm] is actually amazing; I didn't realize how much better it makes the beam chain to different enemies. It seems to affect nearly everything with a distance or radius on this weapon, of which there are a lot. Highly recommend this mod.
Flex Slot
The empty mod slot represents your flex slot. Riven > Bane > [Serration] > Galv Scope > [Hammer Shot] is the priority.
Riven is optimal of course, since [Torid] has a nutty disposition, and Rivens crank its output up like crazy. A basic unrolled [Torid] riven is currently selling for about ~800p right now though, so understandably, you may need other options. Ideal riven stats are Crit Damage > Multishot > Crit Chance == Damage, IMO. Mine's just +CD/Dmg% and it's great.
Bane is next in line; +55% damage is best in slot, especially when it's +140% damage to slash procs (multiplier applies twice in that scenario). Of course, swapping banes is a pain (you might as well be running nothing in that slot if you're facing a different faction), having a full suite of leveled Bane mods is also expensive, and [Torid] is not in need of absolutely optimal damage to function very well, so this is an understandable skip as well.
[Serration] is your best bet if you don't have a riven or bane (or just don't want to use either). It's what I used to have listed in this slot in the build until too many braindead meta parrots kept clowning on me in the comments for having a build with Serration + Merciless. News flash: that's still good. Merciless alone is +0-360%, and adding in Serration makes it +165-525%, which is really quite nice. Even at full stacks that's 35.9% more damage over Merciless alone, and of course it's way more damage before you manage to stack it (85% vs 250% total dmg. that's nearly triple homie). Saving you the pain of stacking merciless is a big perk - and you don't have to worry as much about maintaining those stacks either, since you aren't nearly as reliant on the arcane. (Also, you can run [Amalgam Serration] for bonus sprint speed, another perk of this option.)
[Galvanized Scope] is fun cuz you can get funny red crits (indeed, the first edition of this guide ran it, calling it a "Red Crit Monster"), and it will outperform [Serration] assuming everything is stacked (+320% CC on top of a mere +200% is better than +165% dmg on top of +360%), but getting it stacked is quite difficult (stacks up to 5x off headshot kills) and needing to aim down sights all the time is a pain. You could argue this should be higher up, as it is your most damaging option outside of a riven if you can keep it stacked (and aren't getting a lot of mileage out of Bane slash procs), but it's too finicky in my book.
[Hammer Shot] is bad compared to the other picks here. The status chance is not important for a pure Viral build and the CD bonus is meager (220% -> 280% is a 27.27% damage increase). I've only found use for it on a Corrosive build, as a side-grade to Hunter Munitions (which is much less effective on a Corrosive build than a Viral build).
Honorable mentions:
- Bladed Rounds: +54.54% damage while active, assuming Vital Sense for CD. This is basically just a bad Scope.
- Galv Aptitude: Given we don't really care about status chance, and only get 2 statuses, this is almost objectively worse than Serration on this build. Stacks viral faster, but that's really not needed.
- Other elemental mods: nah, brah. you'd need 3 mod slots to add another element, and Primed Cryo is already a perfect solution. only time this is a good idea is switching to Corrosive for a 14-stack corrosive build, and even that's questionable, given slash procs ignore armor anyway.
- Vigilante Armaments/Fervor: lol no
