Can be any of the dungeon bosses, primals, or the coil bosses (that soon follow up with Calamity Unbound.)

Mine would personally be T11. You step into the Core Override, and you get ambushed The Avatar whom entered the arena via an Allagan Taxi. The thunder SFX is also a nice, silly touch to it. I can't take it seriously, esp with the pose it does at the end.