The Choir of Torments are from one of the Pretenders hosts of Slaanesh.  If they are part of a Slaanesh army, that army must use the Pretenders battle trait.

Profiles:

Battalion Abilities
Fanatical Guardians
Add 1 to the Attacks characteristic of melee weapons used by units from this battalion while the unit is wholly within 12" of the INFERNAL ENRAPTURESS from the same battalion.

Used By (1)

Chaos - Slaanesh(Catalogue)