The sustained spellcaster potion is a unique potion that it is untradeable, and also cannot be made through any method of Herblore. The potion can only be obtained midway through The Poison Project grandmaster quest, and be purchased thereafter. The potion requires 99 Magic to buy and therefore use. I can only be obtained from the Hooded Mystic once every five hours for a fee of 100,000gp.