This is the final form of the boss. NOT the second one in Phase 2 Room 20, but in Phase 3 Room 20. (Basically 60 rooms you need to finish.)
Take note that each further form has an added lifebar. Phase 1 has one, Phase 2 has two and Phase 3 has three lifebars. But let's be honest, if you beat one, you'll beat them all. It's not exponentially harder. Best use you new Disgorger Weapons here. The beam and the explosive ones are great. Shoot the eye, kill it.