Soul Harvest is a Terror Witch Doctor skill unlocked at level 9. This skill was initially a damaging attack, but changed greatly during development. It is now a buff that provides a considerable boost to the Witch Doctor's spell damage.
To use Soul Harvest, the Witch Doctor casts the skill and receives a stacking bonus, from 1-5, depending how many many enemies are near the WD when he casts the spell. The radius of effect is not specified in the tool tip, but seems to be about 15 yards from the Witch Doctor. The bonus lasts for 30 seconds (Soul to Waste can double that to 60), and it can not be recast for 15 seconds.
There is strategy required to use it properly, as the Witch Doctor wants to have at least four or five enemies in range when he uses the skill. Casting this with no monsters nearby grants zero bonus, and with the cooldown the skill can not be recast for 15 seconds.
This skill was initially a nova-type of attack, one that dealt damage and fed mana back to the Witch Doctor for everything it killed. That changed entirely during development, and the skill now buffs the Witch Doctor's spell damage.
I seldom needed it, and found it to be less than effective as a killing weapon. It wasn’t that damaging, and wasn’t that cheap to cast either; so I had to make sure it was going to kill at least a few monsters to be cost effective.
The old version of this skill created a nova-like ring of blue smoke around the Witch Doctor. All targets within the ring were damaged, and if the damage was enough to kill them, their souls (white/blue ghosts) swirled into the air, then down into the Witch Doctor, where they added to his mana. The skill was just the same at BlizzCon 2009.
Soul Harvest was changed entirely for Blizzcon 2010, becoming a Tier 3 buffing sort of war cry. Unfortunately, there are no gameplay reports detailing any usage of this skill from that event.
Skill tiers were removed shortly before the beta started in September 2011, and Soul Harvest became a level 14 skill. Soul Harvest was one of the skills available in the BlizzCon 2011 PvP Arena, and Adivirgi enjoyed using it:
This is a crazy awesome spell that lets you damage multiple enemies and receive a pretty decent damage buff at the same time. Its downside is that it also has a very short range so you basically need to be right in the middle of a pack to get the maximum effect for this spell.
Unfortunately, putting yourself "right in the middle of a pack" is a very dangerous thing to do in the Arena. This skill is much more useful in the regular game, where running into a pack of enemies generally won't kill you instantly.
The skill system was once again overhauled for the Beta Patch 13 in February 2012, and Soul Harvest was classified as a Defensive skill and moved up to level 22. For Beta Patch 15, it was re-classified as a Terror skill, moved back down to level 9, and had its mana cost increased.