Basically, Auric forces the Illians to follow the White Hand. If he's killed, or you kill him, you are able to adopt other religions... But your traits, and access to Auric Ascended, is lost.
To go with this, we can add a new Illian leader (Probably Raitlor from LENA, I like the art) that has access to all religions, but is blocked from the higher level Illian rituals... Basically, everything past the White Hand. If she founds the Hand, and adopts it, she will be able to produce certain rituals, I think everything up to Draw/Ascension (She can't ascend, but you CAN get Drifa or freeze everything). However, if you produce these rituals you are from that point on locked into the White Hand religion.
Could even possibly allow for a leader switch to Raitlor if Auric dies... I think that would make people too willing to loose him though.