Endless Hail is a skill located in the Bow (which can be found in the Weapon skill tree). Endless Hail is a morph of Volley.
Endless Hail deals the following types of damage: Physical 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