A couple of questions about Exhume Memories:

Why is the HD of those who forget based on the subject's HD? From what I can tell, casting this spell on a first-level commoner with no importance to anyone will have no effect. Casting it on a seventeenth-level hero will wipe his existence from everyone except high-level characters. Cast this on the Tarrasque, and all but the gods forget its existence. Cast it on a Xixecal (a powerful monstrosity of ice, the bastard child of ice gods), and even the most powerful of deities will lose their memories of it. The stronger and more influential the creature, the more people forget it. Not to mention that it seems strange that any Wizard capable of casting seventh-level spells can purge the existence of a dead god from history if he only spends an hour working on it and acquires seventy holy symbols or somesuch.