Grim Reaper is a rune effect in Marked for Death, a Discipline-spending supplemental Demon Hunter skill.
This rune effect wears one of the least clear tool tips in the entire game. The description appears to mean that the marked enemy takes 12% additional damage for the 30s duration of the skill effect. In addition to that damage, the same amount of damage is magically distributed to other monsters within 20 yards of the target.
This sounds confusing, but it's basically the same amount of extra damage taken by the first target, spread to others. So if the monster would normally take 1000 damage, this rune would make it take 120 extra, and then another 120 would be dealt to other monsters within 20 yards. That doesn't sound like very much, but imagine if there was just one other monster within 20y; that other enemy would basically share the same curse and take the same damage, even though it wasn't being hit.
In theory a boss could be hit with this, and as it took damage all of the minions (who have much lower hit points) would drop dead without them even being hit directly.
Skill Rune Effects
See the Marked for Death rune effects page for thorough descriptions of all five rune effects in this skill, including screenshots, videos, and strategy tips.
|| Marked for Death
|| Cost: 3 Discipline|
Mark an enemy. The marked enemy takes 20% additional damage for the next 30 seconds.
|| Grim Reaper
|| 20% of damage dealt to the marked enemy is also divided evenly among all enemies within 20 yards.
Screenshots and movies will be added post-release.
Prior to the skill revision in February 2012, this rune effect was enabled by the Crimson Rune, granting the following effect with a level 7 runestone:
|| Grim Reaper
An additional 40% of damage done to the target is also divided among all enemies within 20 yards.