Tezcatcatl was an ancient and powerful aztec warrior demon. He forged a mystical talisman that would harness the power of his sun God, make him "supernova" powerful. But he got found out, was sentenced to die on the Aztec version of day of the dead. He had a shaman put a curse on him to return from the dead every 50 years. Been doing it for centuries. If it consumed the hearts of heroes, or acquired the Aztec talisman, it would gain the ability to remain on Earth permanently.
During its appearance in 1954, it arrived in Los Angeles, where it was defeated by five Mexican wrestlers who had dedicated themselves to protecting their people, although it successfully slew four of them.
Fifty years later, it returned once again to Los Angeles, forcing Angel to recruit Numero Cinco- the last brother- and actual guardian of the talisman, to help him defeat it, despite the fact that both of them doubted their heroic worth; Numero Cinco was bitter at his past few years working for Wolfram & Hart- having started out as hired muscle and now reduced to simply working in the mail room-, while Angel's doubts over his new role as the CEO of Wolfram & Hart were made even worse when the demon rejected his heart (Although his friends suggested that it was his vampire status rather than anything else that caused the demon to reject him).
As the demon attacked Numero Cinco in the graveyard where his brothers were buried, his blood fell on their grave, causing his brothers to temporarily return from the dead, the four collaborating with Angel to pin the demon to the ground long enough for Angel to destroy its heart and send it back to the netherworld.