Avilas was one of the main antagonists of the Buffy the Vampire Slayer episode "Help". Although he didn't personally appear until near the end of the episode - instead his will was served by Peter Nicols and his secret society known as the Cult of Avilas.
Avilas was a large and powerful demon that offered any mortal foolish enough to worship him the gift of infinite riches - however the price for such a reward was a heavy one: human sacrifice - unfortunately for the Sunnydale student known as Cassie a certain Peter Nicols was more than willing to offer up her life to the demon in his pursuit of power, alongside a group of his friends.
Avilas apparently sat in wait as his mortal followers began their ceremony but took corporeal form when Buffy Summers interfered with the ritual, he then personally fought the Slayer for a brief period of time until Spike entered and gave Buffy a helping hand in the form of a lit torch, which she used to set the demon on fire.
As Avilas died Peter Nicols came to his "god" and demanded to know where his wealth was - this angered Avilas, who proceeded to bite Peter's neck before apparently dying.