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 70 ((718 (min) + 1312(max)) / 2 * 4 proc per minute / 60 sec = 70 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.