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I always assumed he would get to a 3++ with grimoire (adding to his natural demon save).

Does instead go to 2++ with his oracle of eternity special rule?

As written Fateweaver has a 5++ (from the Daemon rule) and a 4++ (from Oracle of Eternity). If given the effect of the Grimoire he has a 3++ (from the Daemon rule, boosted by 2) and a 2++ (from Oracle of Eternity, boosted by 2). Under the 'Models with More than One Save' paragraph he takes the best available save.

The Grimoire applies to Invulnerable saves possessed by models with the Daemon rule, not to Invulnerable saves granted by the Daemon rule specifically. If you were to cast Forewarning on a Daemon unit (for instance) to give it a 4++ save the Grimoire would similarly improve it to a 2++.

Grimoire doesn't increase the Daemon rule's invul save. It increases invul saves of a unit with the daemon rule