Dhampir refers to any hybrid of one human and one vampire parent. They do not have the weaknesses of vampires. Their reflexes and senses are that of a vampire, while their strength and endurance surpasses humans. Domasnn is, in fact, a dhampir.
As a late adolescent, Domasnn was recruited by the Dark Brotherhood. They were impressed with his skill, and he ruthlessly carried out contracts for the Family.