Appears during The Lightbearer event.
Damage Taken From Elements
Be careful with the beam, sometimes they do nasty combos. They also retarget easily and will run through fire to get to their target.
They don't pass through energy, so bring some walls or bombs to help clearing the place, be careful when going down, there may be several spawned creatures and it can become a dangerous situation. If it is the case enter through the other stairs, fight your way around and lure with caution to get to the basin.