Revendreth is quite possibly the most... efficient... realm at extracting anima. Perhaps they can lend aid where others cannot.
Know that while souls sometimes return from Revendreth, they are never the same when they do. It is a dark realm, where those the Arbiter deems worthy of a chance of redemption are sent.
Their path, however, is not an easy one.
Kah-Sher will assist you in opening the gateway to Revendreth. Meet him in the Ring of Transference.