Ooze king and queen
Yes, we just completed this last night. We ended up clearing the way to them (was fun).
You bring each of them down to low health and just before they die they go into the ground. Once both are “dead” they combine and create a new boss (I forget it’s name) but he was easy to kill. He’d generate adds all the time but we just ignored them for the most part and DPSed the boss down. He gives a chest when complete. Also get credit for an event (Something about killing Royalty).
Yeah it’s an ooze royalty bonus event. It used to be bugged (killing the queen and king did not spawn the larger boss) but they fixed it about a month ago. It’s sadly not really worth doing.