Having spent more time examining this altar than they liked, they decided some things are best left unknown, and departed, heading east into a room that would turn out to be a former workshop, as well as current home to a pair of Glabrezu.
The pair of Glabrezu attempt to use illusions and charms to trick the party into a deal, however they are able to thwart all the demons’ attempts. Having failed at subterfuge, the demons fall back to their baser nature, and attack. They manage to kill one, and defeat the other who surrenders before death. In pleading for his life, he gives up information about the Nalfeshnee boss, as well as a small piece of the history of Demons/Devils. The party is convinced leaving him alive is too great a threat, so they determine to kill him instead. He attempts escape, hearing that, but fails and is killed.
The party continues exploring the ruin, eventually finding a hall that has been recommissioned for a demon hellscape. And what would be inside? Except an invisible, sleeping Chasme.