delirium
One of many true final bosses of The Binding of Isaac, introduced in Afterbirth+. It initially looks like a giant melting skull with small yellow eyes.
Aside from some basic danmaku, its main ability is to shapeshift into any boss previously encountered in a given run.
Due to the nature of the boss and its lair, Delirium is widely speculated to repesent Isaac's mind falling apart as he suffocates inside of a toy chest he locked himself into.