Rilmani exist to protect the middle ground and correct any imbalances that would compromise it. Cuprilachs are spies and assassins of the rilmani. When an imbalance can be corrected by the "removal" of a specific individual or small group, the cuprilachs are sent to take care of it.
They resemble copper-skinned humanoids, when it suits them.
Sources:
Planescape Monstrous Compendium Appendix II
Fiend Folio (D&D 3.4)
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