Sett Runes explained
There's one set of runes for Sett on patch 13.24 that is the clear favorite for LoL players and pros alike. The best Sett rune paths on the position 
Support are 
Sorcery as the primary path and Inspiration as the secondary path with a pick rate of 70%. Besides picking runes on Sett you should learn how to play Sett and if you are interested in pro matches take a look on how a pro builds Sett. If you want to discuss 
Sett then you should visit the champion subreddit.
Sett runes from primary path
Sorcery empoweres abilities of Sett and increases the available champion resources. Most of high elo players – and our LoL pros in the the major regions – have agreed that Phase Rush is the best keystone rune to run on Sett. Phase Rush offers more flexibility and options to betake oneself in and out of danger, which can be helpful since that's essentially the main playstyle of Sett. Other primary runes on Sett are Nimbus Cloak, Celerity and Waterwalking.
Nimbus Cloak, which grants movement speed upon using a summoner spell, works amazingly well with junglers in particular as they take the shortest cooldown summoner spell, Smite. Celerity boosts all movement speed bonuses to Sett. Waterwalking grants movement speed in the river, which helps Sett get around the map quicker. The extra damage will also help in those all important dragon and baron fights.
Sett runes from secondary path
When it comes to your secondary path, most Sett players prefer to go with Inspiration. Inspiration opens creative opportunities on Sett troughout the match. On the Inspiration path, the best runes for Sett are Hextech Flashtraption and Cosmic Insight.
Having an extra way to decrease the distance with the Hextech Flashtraption will make an engage much easier to pull off. Cosmic Insight grants summoner spell and item haste.
FAQ for Sett Runes
Which is the best keystone rune on Sett?
The best keystone rune on Sett is Phase Rush. This Sett runes setup is then referred as Sett Phase Rush.
What runes does Sett use?
Runes from the paths Sorcery and Inspiration are the most used runes on Sett. For the exact pick rates of Sett runes used, you have to study the rune distribution presented.