Extract of Necromantic Power
The item works on every class that have DoT's - even on Warriors / Rogues. It has an internal cooldown of 15 sec, so that it fires around 4 times per minute (can hit critical).
Without any critical hit, it increases your DPS by around 22 (327 * 4 proc per minute / 60 sec = 22 DPS). Of course you have to add the effect of your own increased crit chance plus the crits of the shadow bolts of the item.
- Patch 3.0.2 (2008-10-14): Added.