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- "Once he was a hero; then he became a monster. A force of nature, untamable, unpredictable and deadly beyond belief. His rage is liable to devastate this world and every adjoining shadowrealm... and then either some humani hero or warrior Elder will be forced to destroy him utterly. At least in the catacombs, I know where he is and I know he is safe."
- — Bastet on Mars Ultor
Mars (Greek name Ares) was the Greco-Roman god associated with war and tactics. According to Sophie Newman's memories from the Witch of Endor, he was called Nergal by the Babylonians, Huitzilopotchli by the Aztecs, and Horus by the Egyptians.
After the Fall of Danu Talis, Mars, still a noble warrior at the time, led the Humani to safety, protecting and caring for them along the way. Somewhere along the line something happened to Mars. It is said that Clarent, the Coward's Blade, poisoned him. And then, shortly after he Awoke his twin sons Romulus and Remus, they killed each other, pushing him over the edge. He transformed from a defender of humani to Mars Ultor the avenger.
His wife, the Witch of Endor, cursed him in his madness and encased him in his own aura. Mars' purple-red aura hardened like cooled down lava. Mars was trapped in the catacombs of Paris. Lying in a big circular room on a stone plinth. His servants Phobos and Deimos (the guardians of Mars Ultor also, one gorges itself by fear and the other delighted by panic) protected and served him. Sometimes during his life he was declared an Utlaga, a wolf's head. He is also the uncle of Hel and the cousin of Odin.
- Immortality: Mars Ultor is Immortal and does not age. Though an immortal may be poisoned or slain in battle, they may live for thousands or millions of years.
Dee and Machiavelli convince Josh to follow them to the catacombs of Paris to have Mars Ultor, imprisoned in the room, to awaken his powers. Mars accepts but states that Josh is indebted to him. However, seeing the strength of his golden aura, Mars decides to give Josh a gift free of charge, which turns out to be his military knowledge. Nicholas Flamel, Sophie Newman, Joan of Arc and the Comte de Saint-Germain arrive to see Josh after he is awakened, but during Mars is giving him his military knowledge. Also, it is stated that he is the father of Romulus and Remus, the founders of Rome. He said that his sons were first the closest friends but the power changed them as their hate, fear and other emotions were also heightened by the Awakening process, and Romulus killed Remus. Afterward, as everyone else evacuated, Dr. John Dee encased Mars Ultor and his satyr guardians, Phobos and Deimos, in hardened bone.
Bastet reveals to the immortal Dr. John Dee that Mars Ultor is her younger brother. She calls him by the name Horus. She states that Mars Ultor is dangerous beyond measure.
In The Warlock, Zephaniah freed him and ask him to kill Dr. John Dee. He joins the others at Tsagaglalal's home, where he receives a jade tablet. Then he reveals to Sophie that he has a connection to her twin that he gained from passing his knowledge to him, and that he knows Josh is on Alcatraz. He travels to Alcatraz along with Odin, Hel, and Black Hawk.
While on Alcatraz with Odin, Hel, and Black Hawk, many of the monsters are escaping. A giant crab comes out with three monokerata, Xolotl, and assorted other monsters. They realize that they have to open Areop-Enap's web coating. Billy and Black Hawk head out with the others to fight the giant crab. It eats Xolotl, which is described that since he has tasted Elder aura, he will be satisfied by nothing else. Odin distracts him by taking off his eyepatch and shooting a kind of laser out of his empty eye socket. Mars distracts the crab by flaring up his aura after Odin and Hel are destroyed because of the aura expenditure, but his aura consumes himself, as it did to Odin and Hel.
- Prometheus: In The Warlock, they make amends.
- John Dee: Angered that he (John) trapped him (Mars Ultor) in a "prison of bone".
- He is the namesake of both the planet (Mars) and the month of March.
- Despite their marriage, it is never mentioned if Mars Ultor and the Witch of Endor had any children together. Therefore, it remains unclear whether Scathach and Aoife are his granddaughters, or if they share any relation to Romulus and Remus.
- It's unclear if Mars was affected by The Change, as it had already affected his sister Bastet and her two sons by the fall of Danu Talis. His vampiric traits and insanity may have been a result of The Change, or a distortion resulting from his continuous use of Clarent.
- In The Enchantress, Mars Ultor is revealed to go by the name Huitzilopotchli prior to Danu Talis' Fall, and doesn't recognize his more commonly used name when it's used. It's possible that Huitzilopotchli is his given name.
- In The Sorceress, Bastet emphasizes that as Horus, he was her younger brother. It might be possible that Horus was his given name, as the Royal family of Danu Talis is largely represented as deities from Egyptian mythology.