The Altar of Zaros.
The Altar of Zaros found under the Digsite, inside the Senntisten Temple (after The Temple at Senntisten). Players can change their Prayer book into Ancient curses and vice versa. Players don't have to return here to switch back to standard prayers, as this can be done at any Altar.
When a player prays at the altar, it restores their Prayer points and gives a +15% prayer boost over their current level, similar to the way the Altar of nature in the Nature grotto, after Nature Spirit, and some others boost the prayer points by 20 above their current level. Given that the requirement for the quest is level 50 prayer, using the altar to boost prayer would increase the available prayer points by at least 70 points. At level 99 prayer, one could boost ones prayer to 1130 prayer points using the altar, or 1138 if one uses the "boost" on the Prayer Skillcape.