ZDoom

Hello,

I've just finished BoA, all secret levels as well, and decided to double-check game resources.
Found a specific boss there, and I'm not sure I faced him ingame.

Its a frankenstein-like boss, one of the sprites show mask, and resource name starts with PHAN.
I think it is an easter egg to Phantom of the Opera.
It wasn't presented in chapter 1 and 2 editions of the game.

As Tormentor667 himself answered in BoA thread, it is a miniboss in c3m0_a mission.
I think my mind just skipped that part, as there was much greater threat on that level.