Cyvus Vail was a recurring enemy of Angel and his team in the series of the same name, he was one of the most powerful demons in L.A. and one of the elite members of the demonic organization known as the Circle Of The Black Thorn.
Cyvus Vail was one of the most powerful beings in Los Angeles, with metaphorical tendrils extending throughout the city, obtaining and ensuring his power thanks to his vast wealth, his magical power and his membership of the Circle of the Black Thorn.
After Angel agreed to take over the Los Angeles branch of Wolfram & Hart in exchange for a new life and memories for Connor, Vail was the leader of the elite team of powerful sorcerers hired by the firm to reshape reality and alter the memories of those involved in Connor's life. Vail kept an Orlon Window, a spell which allowed him to observe reality as it was before the alteration.
In the following months, Vail managed to obtain the Resikhian Urn, in which the demon Sahjhan had been trapped by Justine Cooper. Sahjhan was an old enemy of Vail, and had tried to kill him many times, for which Vail sought to have the prophecy of the Nyazian Scrolls (The one sired by the vampire with a soul shall grow to manhood and slay Sahjhan) fulfilled.
In order to recruit the one prophecized to slay Sahjhan, Vail sent his minions, Kith'harn Demons, to harass Connor's new family, the Reillys. Afterwards, Vail threatened to restore Connor's memories in order to coerce Angel into preparing Connor to kill Sahjhan.
Connor managed to slay Sahjhan, though only after Wesley Wyndam-Pryce smashed the Orlon Window, restoring Connor's memories, including his fighting knowledge. Afterwards, Vail disappeared until Angel was inducted into the Circle of the Black Thorn.
When Team Angel decided to wipe out the Black Thorns, Wesley is selected to eliminate Vail based on his adept skills in magic. Angel also sought to exploit the fact that Vail believed Wesley was unstable enough to make a move for Angel's position.
Vail welcomed Wesley into his mansion but was wary of his intentions. After the ex-Watcher attacked him, Vail unleashed his power against Wesley, eventually subduing him and fatally stabbing him in the stomach. However, Vail had little time to savor his victory.
Wesley blasted Vail with one final attack, knocking Vail out temporarily. As Wesley lay on the ground, dying, Illyria came to his aid. To ease Wesley's death, Illyria took on the form of Winifred Burkle.
Vail recovered and, fooled by the frail appearance of Illyria/Fred, taunted the angry Old One, giving her a free-shot as well. She smashed the old demon's skull with a single punch.