in Arya Iwakura's MAP07, "Baron's Banquet", for the excellent official Doom II addon No Rest For The Living, there's an optional secret area behind the yellow skull key door that ultimately leads to a BFG9000. the secret itself is an only mildly conspicuous red brick wall whose textures are slightly out of alignment with surrounding walls. interacting with the wall causes it to slowly lower, and if you don't happen to immediately unload on what's behind it, you realize: this cacodemon is just looking out the window. it'll aggro if you walk around to the front of its face, but otherwise it's just... chillin'.
i think about this cacodemon from time to time. consider this post a celebration of its existence.
