Necromancy, Thaumaturgy and Abyss Mysticism do not have elder powers or techniques. Instead, practitioners of these arts gain access to mystical rituals specific to their art. Rituals are formulaic and require a significant amount of time, as well as specialized implements and ingredients. You cannot buy a specific ritual until you have purchased the appropriate dot of Obtenebration/ Necromancy/ Thaumaturgy to support that ritual- for example, learning a level 4 Thaumaturgy ritual requires you already possess 4 dots in your primary Thaumaturgy path. In addition, you must purchase one ritual of each level before you are able to purchase a ritual at the next-higher level. For example, in order to purchase a level 2 ritual, an Abyss Mystic must already possess at least one level 1 ritual.
The cost to purchase a ritual is equal to the ritual's level times two. Therefore, a level 3 ritual costs 6 XP to purchase. A Thaumaturgist or Necromancer cannot learn more rituals than dots of Thaumaturgy/ Necromancy that she currently possesses. For example, Marianna Giovanni possesses 4 dots in the Sepulchre Path, her primary path, as well as 3 dots in the Bone Path, and 2 dots in the Ash Path. Thus, she can learn nine Necromancy rituals. An Abyss Mystic is not limited in the number of rituals she may purchase.