Sunn O))) and Boris are an American drone doom band and a Japanese experimental rock/doom band, respectively. They collaborated for the recording of Altar which was released on October 31, 2006 through Southern Lord. In addition to Sunn O))) and Boris, the album also boasts an extensive roster of guest musicians and collaborators such as Kim Thayll (Soundgarden), Joe Preston (Earth, Thrones, Melvins, High on Fire), Phil Wandscher and Jesse Sykes (both of Jesse Sykes & The Sweet Hereafter) as well as longtime Sunn O))) collaborators Tos Nieuwenhuizen and Rex Ritter.
Altar was released in numerous formats: a limited 2CD edition was released October 23 via Southern Lord with a 28 minute bonus track featuring Dylan Carlson, titled "Her Lips Were Wet With Venom." Inoxia Records released their own 2CD version in 2006 which features a bonus track on the first disc, "The Sinking Belle (Black Sheep)" (a second version of "The Sinking Belle (Blue Sheep)"), and also features "Her Lips Were Wet With Venom" on disc two. The 3LP by Southern Lord contains all of the songs from the two-CD edition, additional pictures, and liner notes by Kim Thayll. The Daymare Records 3LP version was released March 23, 2007 and features a bonus track not found on any other version of this release: "The Sinking Belle (White Sheep)," a third version of "The Sinking Belle (Blue Sheep)."
On December 10, 2007, the bands collaborated at the Forum in London along with a number of the guests featured on Altar for a live performance. A 3LP picture disc version of the album, limited to 500 copies, was sold exclusively at the show. The bands have since repeated this collaboration for the first times in the United States at the ATP New York 2010 music festival in Monticello, NY on September 5, 2010 and at the Brooklyn Masonic Temple on September 7.