An Unholy Altar in Quake is a wall that must be shot repeatedly to be allowed to continue. There were a few created by the Monsters that worshiped Shub-Niggurath for sacrificial rituals. Ranger defiled the Altars by shooting them repeatedly (or until the required amount of hit points has been taken down). Splash damage does not injure an Unholy Altar, meaning it is impossible to use the Grenade Launcher to shoot the altar. Monsters can use their projectiles to injure the Altar, yet the Vore has an explosive attack and is therefore unable to cause damage to an Altar. Explosives themselves can injure the Unholy Altar as it is possible for the player to use a Rocket Launcher to defile an Altar. They are only found in E4M6.