Just after doing Sisters of the Moon in Tomb as a shadow priest today, I had an extra action button called Absorbing Elune's Power. Activating the ability triggered a 30 second heavy ticking DoT (roughly 2 million every second). I died to it, as I was in a pug and the rest of the group ran ahead, and the EAB disappeared shortly afterwards. I'm guessing it has something to do with Words of the Old God: Elune's Wrath achievement, but as I died I'm not sure. You'd probably need your entire raid to heal you to survive.

Regardless, I just thought it was interesting and wanted to hear if anyone else has seen this button? For the reference, this was on heroic, and I didn't get it the times I killed them in 2x Normal or the first time on HC. It might also be worth noting that I died during the encounter to a solo Incorporeal Shot.