Borangos the Horror
Borangos the Horror is a Great Rogmul and was one of Sauron's chief servants. While similar to other Regmyl in appearance, this spirit of fire is possessed of a "terrible majesty" which is "an order of magnitude greater than those others."
At some point in history, he was sealed by an ancient ward in Nargroth, the Caverns of Fire, beneath the Mithram Spur. In the Third Age, after the Men of Gondor were driven out of Talath Úrui, construction was started on the underground citadel in Nargroth. This project inadvertently freed Borangos, and though the Gúrzyul in charge, Urudanî Stonemaiden, confronted the Rogmul, she was easily defeated. Borangos did not destroy her, but turned her into a flame-wraith, and turned the Orcs and Uruks serving under her into his Ghâsh-hai.
The Lord of the Nazgûl and three of his fellow Ringwraiths later went to investigate Urudanî's disappearance on behalf of Sauron. Upon meeting Borangos, who had claimed Nargroth for himself, the Nazgûl suggested he swear fealty to Sauron. Borangos agreed, and thus Urudanî and the Ghâsh-hai came back under Sauron's command, albeit indirectly.
After Sauron's defeat, Borangos is one of the major remaining powers in Mordor. However, he seems to have an aloof attitude towards the resulting intrigue, unlike the scheming of his peers. In truth he cares little for the sort of power sought by the others, but rather seeks some source of power under Orodruin which "drives" him.
-  Instance: Borangos The Horror
-  Instance: Fate of the Stonemaiden
-  Instance: Union of Evil