Anthraxus
From Marillia
Anthraxus, also known as Phraxas, is an ultroloth who currently holds the position of urgaloth—the (nominal) leader of the yugoloth race. He is one of the mightiest yugoloths in history and one of the most powerful fiends in the Lower Planes.
He's a bit of a baller. He is the longest-serving urgaloth. He was briefly supplanted by Mydianchlarus, but Anthraxus returned and fucked him up real good.
Power Ranking
Scholars of the Nentir Vale estimate Anthraxus to be level 31 (note: Borys is level 33), putting him approximately at a CR of 21.