If you've been participating in the fight against the Scourge invasion you would know that it is possible, during one phase, to summon a Shadow of Doom that drops 30 Necrotic Runes (for a price of.
The mobs are initially autotagged to the player/party/raid who summoned them, so spamming instant cast spells won't help you steal the mob.
However, there are two ways to steal the summon.
1. Kill the player who summoned it. This is clearly working as intended.
2. The summoned mobs are actually leashed to quite a small area surrounding the crystal that you are meant to be destroying.
What you can do is take aggro on the mob (Taunt, Searing Pain spam etc), and move away from the central crystal until it deaggros and runs back to its spawn point. It is not very far and doable before the enemy reacts. At this stage the mob will be vulnerable to tagging from *any player*, and you win 30 free runes, assuming you can kill it.
Working on Dreadmaul-US as of 26/10.