Levistus

Prince Levistus is the archdevil of Stygia, the fifth of the Nine Hells of Baator. He's been frozen in an iceberg for the last several millenia after he killed Asmodeus's wife, and only recently even regained control of Stygia. Hence, he is known as "the Still Lord." Being frozen, he communicates via telepathy.

Official campaign
It's revealed that Ammon Jerro had made a pact with Levistus as part of his efforts in stopping the King of Shadows, but the details of that pact is unknown. The pit fiend Mephasm is one of Levistus' vassals, and apparently was instrumental in the recovery of the Silver Sword of Gith.