I'm a little surprised to learn that grim reapers aren't already a thing in Guild Wars 2, given how many bodies pile up in Tyria during the average day. That grievous mistake will finally be rectified with GW2's Heart of Thorns expansion, which adds the faceless corpse-stealers to the MMO as an elite specialization for the Necromancer class. (It also adds some other, less exciting elite spec classes.)
There's a video showing off the Reaper below, but before that you need to know that one of their traits is called 'Cold Shoulder'. That's a brilliant name for something that, boringly, only makes "chill last longer, and chilled foes deal less damage to you". The Reaper wields a massive scythe, and an equally massive greatsword, so they're fairly slow at hitting things, all told.
Here's what the devs have to say about the Reaper, from this reveal post on the Guild Wars 2 site:
"This devastating melee brawler is at home in battle when surrounded by as many enemies as possible, cleaving through hapless foes for life-force sustainment. Once the reaper has enough life force, they can transform into a devastating avatar of destruction that leaves enemies frozen in terror."