It also looks kind of like the emblems dropped by the wall of flesh!
Hmm, let's see, we see worshipers, the entrance to the dungeon, the player panicking/dancing, and a glowing orb with what appears to be Cthulhu's face on it. Perhaps they're summoning/resurrecting Cthulhu or maybe even his other eye?
Sorry to disappoint you, but Cthulhu confirmed.