Relentless Focus is a skill located in the Assassination (which can be found in the Nightblade skill tree). Relentless Focus is a morph of Grim Focus.
Relentless Focus deals the following types of damage: Disease 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.