Severin's History
Early life
Severin was a young man living in San Francisco and involved with Clare, who wanted both of them to become vampires, though he apparently was not so sure about it. After searching for months for a vampire that would turn them, Clare ran into Alessandra, a high school acquaintance who had "made the leap" and agreed to sire Clare.
However, due to the destruction of the Seed of Wonder, Clare didn't rise as a common vampire but as a feral, mindless one, that attempted to kill Severin. As he tried to protect himself, Severin's power activated, killing Clare in the process.
Attempting to kill Buffy
Using his powers to kill and torture vampires, Severin searched for Alessandra, only to be recruited by rogue Slayer Simone Doffler. Simone planned on having Severin de-power Buffy Summers, the one responsible for the destruction of the Seed, so she would be able to kill Buffy.
Severin saved Buffy from a vampire in order to earn her trust and lured her to an empty warehouse supposedly filled with vampires, all of which had been drained already. Severin then turned on Buffy and tried to drain her powers, while an anonymous caller informed the San Francisco Police—who believed Buffy to be responsible for Severin's killings—of Buffy's whereabouts.
The two fought as Buffy tried to make him run out of power, only to discover that Severin kept the warehouse's basement filled with "living" vampires to recharge himself. After draining all the vampires, Severin began to feed on Buffy. Spike and Eldre Koh intervened, but were quickly put down by Severin, who then tried to drain Spike as well.
As he fed on Buffy and Spike, Severin was shot three times by Detective Robert Dowling. With the real culprit uncovered, Buffy was cleared of all charges.
Severin was taken to a hospital, where he was approached by Simone. Severin asked her to let him drain some of her power to heal from his wounds, but she refused, since she required her full power to face Buffy.
Trying to prevent the Twilight Crisis
Later, in Los Angeles, the Old One Illyria formed a mystical council consisting of herself, Eldre Koh, D'Hoffryn, and Buffy to deal with Severin. Buffy, Illyria and Eldre Koh found Severin's new lair. When they converged on three different sides, Severin pulled a surprise attack on all three of them with the body of a demon. He outmatched the three of them and claimed the only reason he was saving his power was so he could save the girl he killed. The three were teleported from the building. It became apparent he wanted Illyria especially. Shortly afterwards, he attacked the mystical council and drained Illyria's powers, obtaining the power of time travel in the process.
Afterwards he met up with Simone and they began working together again. They met up with Xander Harris. They realized his feelings for the dying Dawn Summers and said that they weren't so different, that they both lost someone to Buffy's influence. With his new time-travelling powers, Severin declared that, with Xander's help, they would prevent the whole Twilight crisis, including Dawn's illness and the existence of zompires. Xander, reluctant at first, agreed. Simone beat Xander up to make it look like he rejected the offer when his friends asked him about it. However, Severin was unaware that his attempts to stop Twilight would cause an instability that would destroy reality.
Death
The two made their way to the Deeper Well around the same time that Buffy, Willow, and Xander did, with Severin hoping to absorb the residual magic there to gain enough power to create a temporal fold that would revert Twilight. However, Severin's body soon proved unable to contain all of that power, and he was unable to revert time. Illyria, however, managed to convince him to help Buffy by accelerating the maturation process of the new Seed of Wonder that was spawned from Willow's magic, thus saving the world and healing Dawn, with Illyria remaining by his side while Buffy and the others battled the now-vampiric Simone and the Old One Maloker. He ultimately succeeded in aging the Seed to completion and restored magic to Earth, but the resulting magical blast completely vaporized both Severin and Illyria, with Buffy vowing to Willow that Severin's sacrifice will not be in vain.
