Subterranean Assault is a skill located in the Animal Companions (which can be found in the Warden skill tree). Subterranean Assault is a morph of Scorch.
Subterranean Assault deals the following types of damage: Poison Damage. Which means that it gets buffed by the following blue champion point trees: Master at Arms, Mighty, Piercing, Precise Strikes. Also the following red champion point trees protect against this skill: Hardy, Iron Clad.
ESO Skillbook