Seth was born of a mortal woman and the Egyptian god Set. After Set rejected him, his mother took him deep into the desert, broke his legs, and left him there to die. Since he was immortal, he didn’t die, but rather suffered until a family of jackals found him. They later sold him for a pittance to Noir, who made Jaden drain his powers. After the first two days of torture in Azmodea, Seth’s pain and grief manifested into a multicolored swallow. This swallow takes the form of a tattoo on his neck until he releases it to do his bidding. The swallow is what captured Solin in the dream realm many years later.
Seth helped the Malachai Adarian escape from his enslavement. Adarian promised to come back for Seth and free him, but he never returned, leaving him to Noir's wrath. Jaden had lied and said he was the one who freed the demon, but Seth adhered to a personal code of honor and confessed mockingly to Noir. He was tortured for a thousand years.
When Lydia ventured into Azmodea to rescue Solin, he captured her but did not turn her in to Noir. He took care of her and her kindness softened his heart. He refused to betray her when Greek gods went to Azmodea and tortured him for her location. Lydia took him to Sanctuary for healing and his aunt Menyara sensed his presence there. Even though he didn't forgive her for not helping him when he was trapped in the desert as a child, she asked Maahes to guard him. However, even Maahes couldn't keep them safe and the Greek gods attacked them again. Seth figured out that they were after Lydia because she was the key to their destruction. To save her, he pretended to kill her and then surrendered himself to Noir. When she awoke from the spell he had placed on her, she returned to Azmodea, this time to free him. They hid with the new Malachai, Nick Gautier, until Seth's powers returned and he was strong enough to protect them from anyone Noir might send after him in the future. They named their son Ambrose after Nick.
In Time Untime, Seth and Set appear to have reconciled, though the circumstances of their reunion are unknown.