The First Evil

"You think you can fight me? I'm not a demon, little girl. I am something that you can't even conceive. The First Evil. Beyond sin, beyond death. I am the thing the darkness fears. You'll never see me, but I am everywhere. Every being, every thought, every drop of hate--."

- The First Evil

The First Evil, also known as The First and The Infinite Evil, is an unique and malicious enity that is one of the most powerful villains in the Buffy the Vampire Slayer franchise - having influence over events well in advance of its ultimate appearance in the final season of the show in which it would be the chief antagonist.

Origin
"You're everywhere. You're in the hearts of little children, in the souls of the rich, you're the fire that makes people kill and hate, the cleansing fire that will cure the world of weakness. They're just sinners. You are Sin."

- Caleb about the First Evil.

The First Evil appears to be the sum total of all that is evil, older than the Demons or even the Great Old Ones and far more ancient than humans, it has existed before the written word and trancends all reality and dimensions (at least those that appear in the Buffy universe) - it is suggested that it may even have been the very first entity to have existed as stories say that the First Evil lurked in the darkness long before the physical universe was even created and that it shall remain long after the universe ends - despite its seemingly massive power-level few have heard of it and even fewer actually believe it to exist. The only thing older than The First is The Seed of Wonder, the source of all magic in the universe.

The First Evil cannot affect the world on a physical level - only able to deceive, torment and manipulate others, having a deep understanding of human nature since it can copy the thoughts of all human beings who have died. It uses this knowledge to drive others to commit acts such as murder and suicide. Though it once expressed the desire to feel what it would be like to kill a human being by it's own hand.

Powers and Abilities
"Fact is, the whole "good and evil, balancing the scales" thing? I'm over it. I'm done with the moral coil. But, believe me, I'm going for a big finish."

- The First Evil

The First Evil is a vastly powerful entity capable of assuming the form of any person who has died, including vampires and dead people who have been resurrected and it uses these forms to frightening effect in manipulating others throughout the series.

Its only true weakness is that it is unable to deal physical damage, being ghostlike in nature - it is however a master of manipulation and act through its many servants or any creature it may manage to trick into doing its bidding.

Trivia

 * Anthony Stewart Head was not allowed to touch anything on-screen for part of the seventh season, beginning with the episode "Bring on the Night" and concluding in "The Killer in Me" in order to set up suspicion that he may actually be the First. Joss Whedon explains he did this "to make people wonder. Just to have a little fun in the sense of pulling a mystery... an exercise, something to spice things up." Whedon wanted to play off the "creepy" idea that "we don't know where the bad guy is, we don't know where he's coming from. Our trusted mentor could be the bad guy.".
 * The First has appeared as 20 different characters over the course of the series: Buffy Summers (10 times), Warren Mears (4 times), Spike (3 times), Drusilla, Jonathan Levinson, Mayor Richard Wilkins (2 times), Jenny Calendar, Daniel, Margaret, Travis, Glory, Adam, The Master, Cassie Newton, Joyce Summers, Eve, Nikki Wood, Chloe, Betty, and Caleb (1 time).
 * Likewise, the First is the character that has been portrayed by twenty different actors.
 * The First takes the form of all six previous Big Bads (The Master, Drusilla, Mayor Richard Wilkins, Adam, Glory, and Warren). In the season premier he does them all in reverse order of the seasons they are antagonists of.
 * Like the Alliance's Parliament of the Joss Whedon series Firefly, the First Evil, as ultimate and final antagonist of the series, survives. This pattern of sparing the series' ultimate antagonist is not followed with Dollhouse, however.
 * Also, The First is one of two Big Bads of the series not to be killed, the other being Drusilla.
 * The First bares some similarities to Highlander: The Series' satanic demon named Ahriman. Like Ahriman did to Duncan MacLeod and his friends, The First taunts Buffy Summers and her friends. Also like Ahriman in Highlander: The Series, The First appeared earlier in the series before it was in the final season. Both even killed members of the Watcher's Council in their respective universes.
 * What makes them different is that Ahriman is not limited to imitating a dead person like the first, also it can actually cause physical harm to the world, unlike The First. Also unlike The First, Ahriman was never the final main antagonist of the final season.