This is the final room of the first level of the Dungeon of Demon Strata. The party stumble on to what happens when someone summons a devil, makes an ill-considered pact with them and allows the devil to escape to settle a few old scores.
It is a confusing mess from the players point of view but at least there is a handy monster for the characters to take their frustration out on.
(Note, everything is very first-draftish).