The Cursed Altar is a piece of interactive scenery found in the Altar of the Sun within the Forthos Dungeon. The altar is cursed as a result of the player attempting to kill Olbertus, who transformed into a Shaeded Beast and laid a curse on it. When this occurs, a message will appear in the player's chatbox: "The Shaeded Beast curses the altar blocking its connection to the Gods."
Attempting to use the cursed altar will result in the player taking up to 30 damage and their prayer fully depleted. In order to lift the curse, 100 eligible dragon bones must be offered to the sacred bone burner in the Crypt of the Moon (see here for more info).
Once the curse is lifted, a message will appear in the player's chatbox stating "You sense the dark presence from the cursed altar vanish. You will now gain prayer xp from the Bone Burner."