Oppressive lane harass and efficient power-spikes.
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.
+20 Ability Power or +12 Attack Damage, Adaptive
You get free boots at 10 min. but you cannot buy boots before then. These boots give additional MS. For each takedown you acquire the boots 30s sooner.
+5% Max CDR
+5% Summoner Spell CDR
+5% Item CDR
Arcane Comet can really shine on poke champions who are able to proc the effect from a long distance.
TIP: If you're only proccing Comet with Xerath's [Q] your hit rate is going to suffer. Try using [W]s slow and [E]s stun to more effectively deal chip damage in lane.
Manaflow Band is what makes Xerath Support possible. His [Q] and [W] mana costs are extremely high with without Manaflow you're going to struggle poking and harassing in lane.
Try to use [Q] when the Manaflow effect is up so that you get the most out of the mana refund.
TIPS: Always try to auto a minion when your passive is off of cooldown, to further sustain your poking pressure.
Transcendence on Xerath Support lets you hit that crucial 40% CDR in the mid game when paired with Remnant of the watchers and Luden's Echo for massive teamfight damage.
Scorch is the bread-and-butter of Xerath support as it further empowers your poking and oppressive damage in lane.
Magical Footwear is a valuable and safe rune option on nearly all champions (minus roaming supports), acting effectively as a kill's worth of gold at 10mins.
The saved gold lets your opt into more AP damage items earlier than otherwise possible.
If you're playing into a few scary assassins/divers (Zed/Nocturne) Perfect Timing can be a great alternative for minimizing deaths.
More CDR means more spell casts/poke and thus more lane harass, playing up on your strengths.