Xasthur was created in December 1995 in Alhambra, California, after Scott Conner (known by the pseudonym "Malefic") had played with several local death metal groups in Southern California. Initially, the band began rehearsing and recording in a home studio with an unstable lineup. A 10-track tape of these early rehearsals was circulated on the trade scene. While the original of that tape was destroyed, some tracks would later appear on re-releases of later albums.
For a self-released split album with Orosius, Conner was joined on drums and vocals by friend Mike Pardi from the band Draconis (a.k.a. Ritual. Subsequently, Xasthur became Conner's solo project, although Khaija "Blood Moon" Ausar (of Dacon and Crimson Moon) later appeared as a session musician on the song A Curse for the Lifeless (from Nortt/Xasthur) and the EP Xasthur, Mark "M.H." Hunter performed on Defective Epitaph and All Reflections Drained, and Marissa Nadler sang on Portal of Sorrow.
The band's first album proper, Nocturnal Poisoning, was released by the 'Blood Fire Death label' in 2002. 'Southern Lord Records' re-released a remastered double-LP version in 2005. In the years that followed, Xasthur's releases were issued through a variety of labels.
Xasthur released several split albums with other black metal acts such as Acid Enema, Angra Mainyu, Black Circle, Nachtmystium, Leviathan, Nortt and Striborg, while contributing to tribute albums to Burzum, Ildjarn, Judas Iscariot, Katatonia and Manes and appeared on various compilation albums. Additionally, Conner collaborated with numerous bands including the drone metal project Sunn O))), Mord (featuring members of Lugubre), Celestia and Gravesideservice, and participated on the first album by black metal "supergroup", Twilight.
On March 26, 2010, Conner announced the release of Xasthur's eighth studio album, Portal of Sorrow. He also stated that this would be the last album under the Xasthur moniker, as he was dissolving the project citing a lack of motivation, among other issues, also making a point that the project would not be restarted. Under his new project moniker, Nocturnal Poisoning (named after his 2002 album). Under this name, three albums resulted between 2012 and 2014: Other Worlds of the Mind, A Misleading Reality and Doomgrass.
On September 30, 2010, the first and only music video to date under the Xasthur name was unveiled, for the song Walker of Dissonant Worlds from the To Violate the Oblivious album.
On March 5, 2015, Conner announced on the official Nocturnal Poisoning Facebook page that the band were reverting to the name Xasthur. In reclaiming the name, he said: For five years, Nocturnal Poisoning was locked out and denied every opportunity or open door that Xasthur used to have, or would've had. I worked hard at building up both projects, starting both of them from nothing and nowhere, but I'm taking back what's mine. Xasthur doesn't belong to the greedy hipsters that milk the metal business; it belongs to me and it's mine to take back. He also went on to state that the current version of the band (including contributions from Nocturnal Poisoning associate members Christopher Hernandez and Robert Nesslin) would be a continuation of Nocturnal Poisoning's acoustic-driven music, saying, There's no need to rehash old Xasthur songs, the acoustic ones are plenty dark, and sometimes they're not. If you've been listening, reading, thinking and getting it, we could call it Xasthur acoustic/unplugged with, whatever, a country, blues, 'folk', bluegrass, doomgrass or singer/songwriter style and technique in it".
On April 16, Xasthur's first-ever live concert appearance was announced, to take place on June 19, 2015 at the Thirst for Light: Cascadian Summer Solstice II festival at Red Hawk Avalon in Pe Ell, Washington.