Grim Focus is a skill located in the Assassination (which can be found in the Nightblade skill tree). Grim Focus is a base skill, and can be morphed into Relentless Focus or Merciless Resolve.
Grim Focus deals the following types of damage: Magic Damage. Which means that it gets buffed by the following blue champion point trees: Elemental Expert, Elf Born, Master at Arms, Spell Erosion. Also the following red champion point trees protect against this skill: Elemental Defender, Iron Clad.