GELDAURAN, HAD THE ABILITY TO NULLIFY MAGIC

If you're fighting against mages who believe they are god, what better tool to have up your sleeve than the ability to nullify their magic? Bring them to equal ground as the slaves they lord over? It's theorized that Geldauran was an extremely revenge-fueled person, wanting to place the Evanuris off of their pedestal and let the regular people of the Elvhen nation have them. The idea of nullification could also come from the fact Geldauran would refuse "the claim laid on him by those who place themselves above him", indicating that he said no to worshipping the Evanuris and saw them for what they were.

It's very likely that this person had an extremely strong, unbending will, and used that to try and lead other Elvhen slaves out from under the oppression applied to them by the Evanuris.