A couple other thoughts:
A crystal skull, possibly enchanted as a flameskull or even a demilich
A bloodstone dorje (psychic wand) carved with glowing illithid script that floats within its display case, tuned to some psychic spell
A brain golem (perhaps a reskinned flesh golem with a ranged psychic attack), created from an amalgamation of brains from victim races
A collection of rare gems, perhaps ioun stones or holding an imprisoned soul (attunement allows asking the being questions or drawing upon a former ability).