Diablo – How does Grim Harvest work

As someone who has only played Witch Doctor and uses Soul Harvest as a main part of my strategy, I can confirm that the monsters need to be within the radius upon casting.

Note that the max for this effect is 5, which is not hard to achieve. Also worth noting: it is very effective to charge headfirst into a cluster of enemies, harvest their souls, then use Spirit Walk to find your best point to attack from.

It's a skill that doesn't work with the usual cast and cooldown mechanic as other skills do. Instead it will activate in 15% of you getting hit by a monster. You'll see that the skill-icon will turn active, but then you'll have to manually activate the skill.

The time-window for activating the skill is also limited. After some time the skill will become inactive again if you didn't chose to activate it.

While it's different from other skills, I personally like the mechanic and the skill already helped me in several situations (surrounded by monsters, this skill can be some sort of "panic-button" that heals you and deals damage in the same turn).

Update: Also note that the skill doesn't have a cooldown. So if you're getting hit a lot, you can potentially trigger this skill quite frequently. Especially if you enable the "Provocation" skill-rune (available at level 52) which increases the chance of activation from 15% to 30%.