Ursoc's Den is in the [52, 23] area. Talk to Tur Ragepaw. Ragepaw is a druid, and will shift in to bear, moonkin, or tree form when you're ready to begin. He will stay in the selected form for the entire fight.
Spirit of Ursoc says: I thank you all. The druids of old were wise to tear down Vordrassil, for its roots seep deep into the dwelling of an ancient evil.
Spirit of Ursoc says: You know their kind as old gods. Beware Yogg-Saron, the beast with a thousand maws. His evil extends beyond Vordrassil's roots.