Astaroth, Shadowlord of Hatred
Astaroth is the Shadowlord of Hatred, and is one of three such beings that represent the opposing aspects of the Principles of Truth, Love, and Courage.
Astaroth first appeared in Ultima V, terrorizing the townes of Britannia in the absence of Lord British, whom the Shadowlords had successfully captured and imprisoned. He was eventually defeated when the Avatar summoned him to Empath Abbey by calling his name, and casting the Shard of Hatred (from which Astaroth originally sprang) into the Eternal Flame of Love.
The Shadowlords reappeared later in Martian Dreams, in the Avatar's dreams. Astaroth taunted the Avatar with a test of hatred, which in the end was unsuccessful; the Shadowlord had again been bested.
Astaroth has not been seen since.
Trivia
It is in Martian Dreams that the Shadowlords are given genders, and Astaroth was revealed to be male.