Lane poke pressure and back-line teamfight damage.
Empowered abilities and resource manipulation
Hitting a champion with spells hurls a comet that deals adaptive damage at them. Landing more spells reduces Arcane Comet's cooldown.
Periodically your next ability used has its mana or energy cost refunded and restores some of your missing mana or energy.
Gain 10% CDR when you reach level 10. Excess CDR becomes AP or AD, Adaptive.
Your first ability hit every 20s burns champions.
9% attack speed, 10 ability power or 6 attack damage
Takedowns restore health and grant bonus gold.
Deal 7% more damage to low health champions. Gain AD/AP after kills or assists.
Comet's poking nature allows you to repeatedly proc Comet for additional harass damage. If procced with [W] or [E] you're much more likely to Lane the Comet thanks to the slow/stun.
Manaflow combined with Xerath's passive provides you massive mana sustain. Xerath has massive mana costs on [Q] and [W] so make sure to use those abilities to get the refund.
Transcendence allows Xerath to hit 30% CDR with Morello's alone for a massively powerful mid-game poke and harass.
If you'd rather opt for less, but more powerful spell casts, Absolute Focus is a great alternative.
Xerath excels at poking in lane and Scorch is the ultimate lane dominance rune. Combined with Comet, Scorch should easily put your target in Coup De Grace range very quickly.
Triumph is an all-around solid rune choice on any champion. The extra heal on kill/assists allows you to continue on in fights.
Coup De Grace is the go-to rune for mid-lane mages who are looking to pump out maximum damage to back-line targets.
Xerath's long range allows him to hit the back-line no matter where he is positioned, making him prime for taking full advantage of Coup De Grace.