After many hours of trying Sevagoth in field, like SP Plains, and various SP survs this suits the best my vision of Sevagoth, which is as it is - He is glass cannon, which means that the best way of defense is an offense; thats why i don't try to compensate his eHP over stats, because 1k more eHP won't make a difference in SP, maybe it does in low level missions like sortie for ex.; but that's another story.

Anyway, this build allows u to throw some serious dmg without spending too much energy, and it works in SP as good as it does in ESO.

There is a reason that DUR is negative; when u cast Sow (2), there is always chance u won't be able to kill single enemy with combo while clearing hordes, and as long as he is affected by Sow (2) u can't recast it, thats why i intentionally put negative duration, and no, it does not affect efficiency of gloom, atleast not the way that u will notice.

Arc. [Rage] is my personal preference for building up DPS with [Proboscis Cernos] which i also use for stacking enemies, when im going to blow 'em up with Reap & Sow.
Arc. Aegis is just [Lucky Shot], if u get it its great, but if not... well, u would die anyway... but u don't cuz passive... its kinda complicated, but it's not so just don't die, ok?

U may also swap Aegis/[Rage] for Energize but it's not necessary with P. [Flow] and 160 EFF.

Thats all i think,
Later i;ll post build without Umbra Forma.