Brush up on your Greek mythology with this article about the twins, Deimos and Phobos. Deimos is the god of terror, and Phobos his twin brother is the god of fear. They were said to be the sons of Ares (god of war) and Aphrodite (goddess of love). The twins would accompany their father in battlefield, having the reigns of his chariot. They would spread fear throughout the battlefield. Like their father they liked war and pain.
Both brothers were said to be worshiped by the Spartans. It was Plutarch, who wrote that Alexander the Great on the eve of the Battle of Gaugamela, prayed and sacrificed in the name of Phobos. In mythology, Hesiod wrote that Hercules had a depiction of Phobos on his shield, as did Agamemnon . On the modern monument of King Leonidas, his shield has a depiction of Deimos.
According to myths the twins would be the nephews of Enyo, Hebe, Eileithyia, Hephaestus, and Eris (though some legends say that she is the daughter of the goddess of Night). Their grandparents would be Zeus and Hera, and it depends what Aphrodite myth you read, to know how she came about.
The word phobia which you probably guessed it, derives from Phobos. The Ancient Greeks blamed their fears on the son of Ares and Aphrodite. Deimos on the other hand means dread.
In art work they were depicted as youths. Phobos was sometimes depicted with a lion head, which that vision of him, was used on warriors shields, as was stated with examples before. The two of them have moons named after them, when Asasph Hall discovered the moon of Mars, he named them after the twins.
So that's a little bit about these terrifying twins that were once worshiped by the Ancients, and now are just interesting mythological stories that still bring interest to the modern world.