The Holy wrench is a reward for the completion of the Rum Deal quest. If players have it in their inventory when they drink a prayer potion, super prayer potion or super restore potion, they will regain additional Prayer points, depending on Prayer level. The increase is 1 prayer point for each dose drunk of a regular prayer, and 1-3 points per dose of super restore potion depending on prayer level. If a player loses their holy wrench, they can obtain a new one by talking to Davey on Braindeath Island. Having more than one holy wrench in your inventory does not further increase prayer gain.
Some players find the wrench rather useless, as the wrench takes up an inventory space which could instead be used for a Prayer potion. However, with level 50 prayer, having a full inventory of prayer potions (4) will restore 2,128 prayer points (4 * 19 = 76, 76 * 28 = 2,128), while having 27 prayer potions (4) and a holy wrench will restore 2,160 prayer points (4 * 20 = 80, 80 * 27 = 2,160), or 32 more prayer points. Plus, the wrench is technically free, while prayer potions can be very expensive. At level 84 prayer the amount restored by prayer potions is greater without the wrench. It can be used effectively if the player doesn't bring an inventory completely full of prayer potions, or when prayer potions are used with food. It is also useful in circumstances where inventory space is not at a premium, and the space for the wrench can be easily given up.