For this build I use 1 Umbral Forma and 5 standard Forma, but it is definitely not necessary to use an Umbra forma. [Umbral Fiber] is replaceable by [Adaptation], but it's my personal preference to use it with his shadow as he gets survivability innately instead of a buildup, along with the small bonus from the other Umbra mods.

[Steel Charge] is the standard Aura for [Sevagoth's Shadow]. Following that, I find [Preparation] to be a necessity on his shadow as spawning it grants full energy, and enough energy to efficiently use abilities during the duration of this. [Sevagoth] needs at least 200% ability strength as it grants a 100% damage vulnerability on enemies. 175% Range helps for his pull ability in terms of consistency, and it allows for all of his abilities to be used more effectively. I opt for the extra bit of range.

Flow/[Primed Flow] helps cover any energy issues and combined with [Preparation] it is a huge amount of energy. [Cunning Drift] is a personal preference for the extra range and slide speed for movement, but that mod slot is based on your personal preference, and anything can be used. Flow/[Primed Flow] isn't a necessity and you can invest elsewhere like in [Natural Talent] for Cast speed, or duration if you feel the current duration time isn't large enough. If you do that, then I recommend removing [Preparation] for [Rage] for Energy sustainability.

In terms of Arcanes, Arcane [Fury] is always up due to [Sevagoth]'s Claws always doing critical strikes, so a raw 180% damage boost on his claws is massive. It allows for a sustained DPS of over 90k. The second arcane slot is based on preference, but Arcane Energize synergizes well with [Sevagoth]'s main body as it provides energy to him for when you exit Shadow form. Arcane [Guardian] is a choice, but with the high armor already in the build, it isn't completely effective. Arcane Grace is okay, but why bother with it when Gloom heals you to full in a single-use when you are lower on health. Arcane Strike is an offensive choice, but with my Shadow claw build the attack speed isn't completely necessary. Arcane Phantasm is a super underrated choice as Sevagoth often blocks attacks from the front, and the bonus speed feels really great on a melee frame.

Shadow Form
https://overframe.gg/build/205683/sevagoths-shadow/yet-another-endgame-sevagoths-shadow-build/

Claws Endgame Build
https://overframe.gg/build/205694/shadow-claws/yet-another-shadow-claw-endgame-build/

Claws Endgame Build V2
https://overframe.gg/build/205701/

Claws More normal build
https://overframe.gg/build/205696/shadow-claws/yet-another-shadow-claw-endgame-build-alternative/