Altar is a collaboration album between Japanesedoom band Boris and Americandrone doom duo Sunn O))), released on October 31, 2006 through Southern Lord Records. A limited two-CD edition was released on October 23 via Southern Lord with a 28 minute bonus track with Sunn O))), Boris, and Dylan Carlson, titled "Her Lips Were Wet with Venom". Inoxia Records released their own two-CD version in 2006 which features a bonus track on the first disc, "The Sinking Belle (Black Sheep)", and also features "Her Lips Were Wet with Venom" on disc 2. The triple-vinyl edition by Southern Lord contains all of the songs from their two-CD edition, additional pictures, and liner notes by Kim Thayil. The Daymare 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)".
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 this London show.