Angels & Demons
As bad as a fallen Paladins of Lir might be, they are nothing compared to a corrupted Paladin, commonly known as Blackguards. These men do not give up their Divine powers, they trade them willingly and knowingly for evil powers. To accomplish this they typically ally themselves with a powerful Demon or Devil who arranges and oversees the arcane ceremonies needed to complete the decent from on high into the lowest depths of the pit.
Blackguards are nearly always found working alone or leading organizations, and they are inevitably tyrannical and demented leaders. Many of them work actively to broaden the reach of the evil that they represent, while others seem to exist only to torture and destroy any good they come across, whether it advances a larger agenda or goal.