Astaroth, from Martian Dreams
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. Appearing first in Ultima V with its siblings Faulinei and Nosfentor, this entity later recurs in Martian Dreams, manifesting within the Avatar's psyche while in a dream machine.
Astaroth came into being when the maddened Captain Johne slew his companions in the underworld, inadvertently mixing their blood with three shards of the shattered Gem of Immortality. Brought into the world through one of these imbued shards, this embodiment of hate quickly joined the ranks of its siblings, assisting in the capture of Lord British, and the following corruption of his regent, Blackthorn. With Britannia's governance thus disrupted, Astaroth was free to ravage the eight Cities of Virtue, spreading its aura of antagonism and strife wherever it passed.
However, Astaroth's power was far from absolute. Among his minions was a gargoyle known as Sin'Vraal who, during his service to the Lord of Hatred, was able to contact the captive British, learning from him the ways of virtue and eventually coming to leave his master as a result. Later, within his isolated home in the Drylands, the penitent creature revealed to the Avatar the true name of the entity which he once served, which the hero thereafter used to lure it into the halls of Empath Abbey, where the hero banished it from the world by casting its formative Shard of Hatred it into the Flame of Love.
Years later, within the distant cities of Mars, the Shadowlord reappeared in the midst of the hero's machine-induced dreams. Within these visions, the eidolon mocked the Avatar, claiming that his principle burned still within the champion's heart. Exhorting the hero to accept this, and to harness the unbridled and effortless impetus that hatred brought, the dark being tried the Avatar's dedication to Love in a test of the mind, showing them an injured Martian torturing those they held dear. When the Avatar defied the urge toward vengeance by healing Tarpch, their friend's captor, Astaroth vanished, and has not been seen in the material or subconscious planes since.  
- Astaroth identifies as male in his appearance in Martian Dreams, although his gender is not demarcated in Ultima V.
- Astaroth (taken from the Caananite Ashtoreth) is the name of a prominent figure in demonological systems. Appearing in both the Lesser Key of Solomon and the Dictionnaire Infernal, the demon Astaroth is associated with seductions of laziness, vanity and rationalized philosophy. He has been depicted as a crowned naked man with feathered wings and a serpent in his hand, and is said to be the nemesis of St. Bartholomew.