I play Xerath pretty often and I have a few suggestions for your build.

Athene's Unholy Grail is almost always the best item to rush. Mana Regeneration and Cooldown Reduction are amazing stats as they allow you to poke and wave-clear with spells very often.

Deathfire Grasp is under no circumstances a viable item on Xerath. The stats are great but easily replaced by other items ( Athene's Unholy Grail for CDR and Rabadon's Deathcap for massive AP). The active is far shorter range than your other spells so it'll be much harder to use, and without a gap closer, putting yourself in that close of range is a huge risk that you normally don't want to take.

Rod of Ages is unnecessary on Xerath.

You're never in range for Abyssal Mask, unless you go in close which as I said is both dangerous and not what you do as him, a long ranged poke champion.