History

Deimos was the son of Ares and Aphrodite. He mainly appears in an assistant role to his father who causes disorder in armies and does not appear as an independent figure in mythology. In the Iliad, he accompanied his father, Ares, into battle along with the Goddess of Discord Eris and his twin brother Phobos (fear). In Shield of Herakles, Phobos and Deimos accompany Ares into battle and remove him from the field once he is injured by Herakles. In Nonnus' Dionysiaca, Zeus arms Phobos with lightning and Deimos with thunder to frighten Typhon. Later in the work, Phobos and Deimos act as Ares' charioteers to battle Dionysus during his war against the Indians.

On the modern monument to the battle of Thermopylae, as well as the one at the city of Sparta, Leonidas' shield has a representation of Deimos.

The god’s Roman equivalent was Formido or Metus.

Deimos is the namesake for the smaller of Mars’ two moons, which was named by Asaph Hall.

In DC:

In the pre-Crisis universe, he and his brother Phobos and aunt Eris, joined Mars in attacking the Amazons of Paradise Island. They almost succeeded in destroying them before Wonder Woman and her allies turned the tide.

In the post-crisis universe, Deimos was the son of Ares, born in the brief union of the War God with the goddess Aphrodite, Deimos and his brother Phobos dwelt principally in the Netherworld, from which they intended to aid their father in the destruction of the Earth, and thus the hated Amazons, as well as the other Olympian Gods. He was serpentine in features and his hair and beard were actually serpents whose venom instilled terror in victims. He was known as the Master of Serpents and was the favoured son of Ares. As such, there was sibling rivalry between the brothers.

At first Phobos attempted to slay the Amazon Princess Diana, who had been chosen by combat and the Gods to oppose Ares' plans, by bringing to life the creature called Decay, but the Amazon Princess destroyed Decay after a savage battle across the city of Boston. At length, the Princess Diana, accompanied by four humans who had befriended her, brought the battle against Ares to the Netherworld itself. Here she confronted Deimos and Phobos, and a struggle to the death began. During the course of the battle Deimos attempted to poison the Amazon with the venomous bite of his serpentine hair, venom that would fill the Princess Diana with blind, unreasoning terror. In the last seconds before she could succumb to the venom, Diana hurled her razor-sharp tiara at the gloating Deimos, and decapitated him. The cowardly Phobos, fearing for his own life, fled into the darkness, swearing vengeance on the Amazon Princess.

Years later Phobos descended into Tartarus to revive Deimos and their sister, Eris, in a plot to regain support from their father. Circumventing the godly edict, to directly interfere with mortals, Phobos selected human hosts for them. His choice of the Joker for Deimos, seemed inspired to begin with as they began their assault in Gotham City. In truth the Jokers own will power and sheer insanity found Deimos fighting to keep control and left his attention compromised. Together, the villains battled Wonder Woman and Batman and their allies. However, Ares soon appeared, embarrassed by what his children had done in his name. Not to mention the plot to poison him and take over themselves. Ares separated Deimos and Eris from their hosts and sent them back to Tartarus.

In Marvel:

Nox, being ever-resourceful and clever, posed as the goddess of love and beauty, Aphrodite, and seduced Ares, the god of war. Nox, according to her plans, later gave birth to two sons, Deimos, the god of terror, and Phobos, the god of fear. Deimos and Phobos were raised by Nox to do her bidding, and were both sent to apprehend Thor and Hercules. In the battle, both of the brothers were killed.

Not being able to live with the death of her sons, Nox utilized the power of human fear during the Great Fear takeover by the Fear Lords in order to resurrect her sons. They held Clea and Rintrah in their captivity, but were both again killed due to the lack of fear in the humans caused by Doctor Strange, this time at the hands of their prisoners.

Other media

Wonder Woman (animated film)

Deimos appears in the Wonder Woman movie voiced by John DiMaggio. He's seen fighting against the Amazons in the battle of the beginning and was later send to fight Wonder Woman after Ares escaped. He is beaten by Diana and tied with the Lasso of Truth, but he is bitten by one of his own snakes before he could reveal any of Ares' plans.