We've been covering the news about Tenris Mirkblood, a temporary Scourge boss hanging out in Karazhan. Adam even has a handy strategy guide that will help you bring him down. The boss is pretty neat, dropping both a non-combat pet and the ultimate in two handed guitars -- the Arcanite Ripper.
Tigole warned us that the boss would only be around for about 10 days. As of just now, Tenris is still in Karazhan, waiting to dole out his badges, batlings, and axes. Now's the time to get in there, folks. We can't gaurantee how much longer he's going to be waiting for you to come kill him. The Scourge invasion reset and started again after it previously closed down, so this is probably as close to a "second chance" as you're going to get.
As a tip, be sure to pick up your Vampiric Batling, even if you already know the spell. The "Already Known" item sells for 10 gold to a vendor. Killing Tenris is at least worth a daily quest to you.