Thymos is the DemiTitan creation of Prometheus by combining the DNA of Kronos and Rachel Dare. He was killed shortly after his creation by Tsumi Tanake.
After the Second Titan War and the death of Kronos, Prometheus came back to Mount Olympus to collect Kronos's scythe and a small vial of his ichor.
Back at Mount Othrys, Prometheus set to experiments with the ichor. He duplicated it, then injected the substance into monsters, demigods, mortals, and even some of the DemiTitans that lived in the mountain itself. But soon, Prometheus realized a true-sighted mortal would be needed, and to fulfill his delusions of grandeur, he went after Rachel Elizabeth Dare. Thymos commanded the DemiTitan army for a short time, leading them into an all out attack of Camp Half Blood. After the failed assault, he was killed by Tsumi Tanake, and Tsumi banished him to Tartarus, with the now dead James McAim as his prison guard.
Owning the Core (made by Illoras, granted permission to by Guywithafedora)
WARNING: SPOILERS AHEAD
After Melinoe returned James to the surface to work for her as a spy, Thymos was left unguarded, only held in Tartarus by his chains. The Overlord then approached Thymos with the Siopilos Core, bestowing it to him and augmenting Thymos's powers. After acquiring the Core and replacing his own heart with it, his ichor is now replaced by the Blackening fluid, his skin has become paler, irises half obsidian-black and half gold, and new Nightmare Armor.
The Overlord also grants him the Sword of Nightmares, Efialtis, later forged with Osveta to form Akyrosei, the Sword of Darkness. Using both Efialtis and Osveta, Thymos single-handedly usurps Hades's throne and escapes to New York with his new undead and demonic army and fulfills Kronos's final wish; to have Olympus torn down brick by brick(when he had the trash cleared away by the zombies he had them spray painted black and used the same bricks and stones to rebuild Mount Othrys), forcing the gods to escape to the crumbling Labryinth. He now rules from his obisidian throne on Mount Othrys, and ordered the extinction of demigods in the country, sending out his personal death brigade, simply known as the Militia, to exterminate demigods.