Re-post to the PTS forum...
Grim focus replaces haste and procs a large hit (bigger than snipe) after 8 light or heavy attacks.
There is a huge problem with this skill in pvp though. Light/heavy attacks against enemies that use damage shields do not count towards the 8 count if they do not break the shield and land actual damage. I spent several hours dueling and against opponents that stack shields i was unable to ever get grim focus to proc over the course of a 1-2 minute fight. In my hours of dueling I never once got grim focus to proc against any opponent that used a damage shield, and I was intentionally trying to get it to proc.
Shields are extremely popular in pvp and light attacks will almost never do enough damage to drop the shield and cause any damage. You might get a few actual damaging hits in, but not 8 in the 20 second time window.
TLDR: Grim focus is worthless in PvP against anyone using a damage shield, which is almost everyone.
Mostly Harmless: PvP leader and officer
mostly-harmless-guild.com