The Soul Glutton was a tentacled demon who murdered and ate humans in large quantities to satisfy his hunger for humans souls. His consumption of souls gave him the power of virtual invicibility and also resulted in him growing to a gigantic size
In the late 14th century, the Soul Glutton was a greatly well-known and feared figure in Aztec society. The Soul Glutton also had a wife and son. The Slayer at the time traveled to the Soul Glutton's native hell dimension with the intent of killing him and its family using an echanted sword that drained the target's energy. The Soul Glutton survived the attack while his family died and his last human slave escaped.
The Soul Glutton swore revenge on the Slayer. However, as time passed much slower in the hell dimension than it did on Earth, the Aztec Slayer was already long dead. Nevertheless, he declared his vendetta would pass onto the next Slayer to cross paths with him.
By 2006, he was much less powerful when he attended a Halloween party on Sunnydale's ruins. He was attracted to the magical vibrations emanated by the Slayer Handbook when it was stolen by Andrew Wells in order to ressurect Tara Maclay. During a confrontation with the Scooby Gang, he regained some of his previous strength and great size due to the deaths of three party-goers at the party and Andrew's ritual.
However, Willow Rosenberg interrupted the ritual after Rupert Giles told her that if the Soul Glutton were to absorb the soul of a resurrected human, the demon would become invincible. Buffy Summers managed to best him by beating his head, causing him to fall into the ruins. Realizing that this Slayer had allies who were also powerful, he decided to refrain from eliminating her.
Buffy then requested that D'Hoffryn locate him magically as he was particularly dangerous. However, the demon appeared to have magical protection. Soul Glutton then made a pact with The Mistress and The Sculptor to prevent the Slayer and D'Hoffryn from changing the Rules of Magic with the Slayer Handbook. For many months, the trio remained hidden, preparing a sinister plot to open all interdimensional portals so the demons native to them would invade the Earth. Once The Sculptor was killed, the Soul Glutton and the Mistress tried to trap the Scooby-Gang into an Hell Dimension. D'Hoffryn killed the Soul Glutton when he absorbed all of his souls which shrank him down to a tiny size. D'Hoffryn then squished him with his foot.