How do clerics gain spells?
Firstly, clerics are not bound by the prerequisite system that mages must follow, except in one regard: required levels of Magery are treated as levels of Power Investiture. If a spell requires Magery 3, it cannot be acquired by a cleric with Power Investiture 2. However, to balance out this freedom, any time he wants to learn a new spell from the list his god offers, the cleric must petition the god for it. This is usually done through a vigil: the cleric must spend at least 24 hours in meditation and prayer — without recourse to the Vigil spell, if he knows it. When he manages to complete the vigil, a roll is then made against IQ + Power Investiture (modified by various things, mostly how well the character has been roleplayed and how well he has served the purposes of his god). If the roll succeeds, the god grants the spell to the cleric. Alternately, a cleric may petition his god in a moment of need for the knowledge of a spell needed right then. If the need is great enough, the g