The identity of the mysterious voices.

[Riftwolf]

They also didn't give O-Chul or Lien an antidote, even though they clearly want to take them alive and talked about it taking two minutes for the poison to kill.

Probably a bluff.

Though that would speak against them being Fair Folk who are bound to be true to their word, as "keep the toxins from burning through his veins" is ... well, debatable. Can you keep something from doing something it would not have done in the first place?

I doubt it's fey/fae/fair folk/fum; it feels like an unnecessary complication when exactly the same role could be played by chaotic outsiders, humanoids, oathspirits or Muppets.

A thought that cropped up was possibly Manticores. They're intelligent, can fly, attack with poison darts, and it'd make some sense for them to be left in charge of a dungeon of super-monsters.