Slay Sarilus Foulborne and bring his head to Sentinel Velene Starstrike in Silverwind Refuge.
There's only one undead mage in the Barrens capable of defiling this many water elementals. He's also the only one foolish enough to try. Sarilus Foulborne is probably sitting at the top of Dreadmist Peak, laughing at me.
[player], you must go into enemy territory and destroy this miscreant. I have had enough of his idiocy and we must show him what happens to those who annoy the Sentinels.
Destroy him and bring me his head. Be wary though, the Horde may be watching for you.