Tarquelne- still mulling over the flavor reasoning. It needs to fit within the RG theme, naturally. I'm currently favoring the following:
Wizard Slayers long ago learned how to empower themselves through the destruction of the Gems of Creation. Each mana node destroyed left behind fragments which they were happy to take back and study, learning more and more of how the unnatural on Erebus ticked.
Part of this research was archived by the Librarians. One philosopher had an intriguing idea. If one of these mana fragments could absorb some of the energies released by a node being destroyed, it could become a powerful weapon. Unfortunately for Biar the Exploded, it turns out pumping a fragment full of unstable energy simply caused a very large boom, a sonic shock wave and the destruction of the surrounding landscape...and librarian.
Lum the Unlucky had a different hypothesis. The Gods saved their chosen champions by bestowing a portion of their spiritual power onto the Immortal. This would be ideal for the RG. Just one problem - spitting in the face of every god ever had somewhat narrowed their access to spiritual power. But, if one trained in the proper rituals killed an emissary of a God with a Mana Blade, they could capture for a time the essence needed to duplicate the feat. Of course, if the RG ever succeeded in its task this power would be lost forever, but a nation of pragmatists rarely worries about 300 turns from now.
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That's the current working idea, if I can lore-ify it I'll include it.