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> Spiky chains lash a fifteen-foot-diameter, floating blob of quivering flesh to a 20-foot-tall wrought-iron scaffold. Two fiends wrapped in chains stand atop the scaffold, torturing the bloated creature's flesh by tightening its chains. Demon ichor oozes from its wounds, forming a shallow pool around the scaffolding.
>
> A third, jackal-headed fiend uses a bronze horn to yell loudly at the bloated prisoner in multiple languages. It sits cross-legged about half-way up the scaffolding.

All three fiends are immune to the sibriex's Warp Creature power.
In the pockets of Fetchtatter's bile-stained robe are two potions of greater healing and six soul coins. The arcanaloth also carries a grimy spellbook bound in chasme flesh. Fetchtatter's spellbook contains all the spells the arcanaloth has prepared.

Each chain has AC 19, 33 hit points, a damage threshold of 10, and immunity to fire, poison, and psychic damage.

Fetchtatter offers 3 soul coins to get the Sibriex to talk, wanting to know about Weslocke so he can kill her and take her position.

The Sibriex offers information in exchange for its freedom. 
It knows info about Weslocke, Bathym, or other Avernus locations.

Fireball
Wall of Fire
Dimension Door
Counterspell