Devil

During the Dawn War, the Exarch Asmodeus took advantage of the confusion to slay the Mourned God. Those Angels that followed him into his rebellion expected to become angels to the new god Asmodeus, but, instead, they were cursed by the gods with monstrous forms to match the monstrous nature of their crimes. They were imprisoned in Hell alongside their rebellious captain. They became devils, cunning, treacherous, and ready to corrupt and tempt mortals to sell their souls to the service of Hell in exchange for power during their mortal lives.
Many scholars believe that the reason for Asmodeus' rebellion may have been to seize divine control to more directly wage the Blood War  against the Demons of the Abyss. If this is the case, we may have the devils of Hell to thank for the many worlds not being overrun already.