Xerath Runes explained
There's one set of runes for Xerath on patch 15.3 that is the clear favorite for LoL players and pros alike. The best Xerath rune paths on the position 
Mid are 
Sorcery as the primary path and 
Precision as the secondary path with a pick rate of 52%. Besides picking runes on Xerath you should learn how to play Xerath and if you are interested in pro matches take a look on how a pro builds Xerath. If you want to discuss 
Xerath then you should visit the champion subreddit.
Xerath runes from primary path
Sorcery empoweres abilities of Xerath and increases the available champion resources. Most of high elo players – and our LoL pros in the the major regions – have agreed that 
Arcane Comet is the best keystone rune to run on Xerath. This keystone rune adds more damage to your abilities and increases your power in the laning phase. Other primary runes on Xerath are 
Manaflow Band, 
Absolute Focus and 
Scorch.
This is usually followed up by taking Manaflow Band for additional mana and mana regenaration. Absolute Focus grants additional damage while Xerath is alomst full health. Scorch increases your control in the early game.
Xerath runes from secondary path
When it comes to your secondary path, most Xerath players prefer to go with Precision. Precision improves attack and sustained damage on Xerath troughout the match. On the Precision path, the best runes for Xerath are 
Cut Down and 
Presence of Mind.
If the enemy team consists of multiple tanks, then the rune Cut Down is the best choice. Presence of Mind helps Xerath with the mana problems by restoring missing mana, if a champion is killed.
FAQ for Xerath Runes
Which is the best keystone rune on Xerath?
The best keystone rune on Xerath is Arcane Comet. This Xerath runes setup is then referred as Xerath Arcane Comet.
What runes does Xerath use?
Runes from the paths Sorcery and Precision are the most used runes on Xerath. For the exact pick rates of Xerath runes used, you have to study the rune distribution presented.