Xasthur was created in December 1995, in Alhambra, Calif. (U.S.), after Conner 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 line-up. A 10-track tape of these early rehearsals was circulated on the trade scene. While the original of that tape is now destroyed, some tracks appear on re-releases of later albums. For the self-released split album with Orosius, Malefic was joined on drums and vocals by a friend from the band Draconis, who used the stage-name "Ritual" (first name Mike, last name unknown). Subsequently, Xasthur became Malefic's solo project, although Khaija "Blood Moon" Ausar (of Dacon and Crimson Moon) appeared as session musician on A Curse for the Lifeless and Xasthur. Before releasing its first official full-length album in 2002, recordings were distributed in limited editions, which were later re-released by the Swedish Total Holocaust Records and other small underground labels. The first album Nocturnal Poisoning (recorded April-September 2001) was jointly released by independent record labels Blood Fire Death and Red Stream in 2002. Southern Lord Records re-released a remastered double-LP version in 2005. In the next years, Xasthur's full-length albums were released through labels such as Displeased Records, Moribund Cult, Total Holocaust Records, Battle Kommand Records and Hydra Head Records. Apart from demo and EP recordings, Xasthur has released several split albums with other black metal acts such as Acid Enema, Nachtmystium, Angra Mainyu, Leviathan, and Nortt. Xasthur has contributed to tribute albums to Ildjarn, Burzum and Judas Iscariot, and appeared on various compilation albums. Additionally, Malefic has collaborated among others with the drone doom metal project Sunn O))) and the black metal "supergroup" Twilight.

The name "Xasthur" is a combination of "Xastur" and "Xenaoth". Malefic explains that he found the former name in a Necronomicon paperback and believes it refers to "a demoness who kills people in their sleep." The latter name refers to a celestial deity he read about in a book on the Afro-Caribbean religion Santería. An original idea for the name was Xasthuriath, but was then shortened to Xasthur in 1997 by the time of the release of a now-destroyed rehearsal tape.