Trapped in a Ritual Altar Sequence

I've killed all the mobs but the Altar won't complete. Searched around and AoE'd the area to try to flush out any hidden mobs, but to no avail. This is not a frequent problem, but also not the first time I've been in this situation.

Please fix.

P.S. This bug occurred on the Sump map. Had to port out to move on.