Read the Tainted Parchment and speak to Nartok inside the Den in the Valley of Trials.
Ah, while you were gone a parchment came for you, [player].
I usually avoid dealing with warlocks if I can help it, but I think you should read it when you have time. If I'm not mistaken, it came from your newly-appointed trainer Nartok. He would have words with you when you're ready.