Swain Runes explained
There's one set of runes for Swain on patch
11.19
that is the clear favorite for LoL players and pros alike. The best Swain Runes on the position 
Adc are Precision as the primary path and
Domination as the secondary path with a pick rate of 31%. Besides runes you should take a look on Swain Build and if you are interested in pro matches take a look at Swain Probuilds. If you want to discuss
Swain then you should visit the champion subreddit.
Primary path on Swain
Precision improves attack and sustained damage on Swain troughout the match. Most of high elo players – and our LoL pros in the the major regions – have agreed that
Conqueror is the best keystone rune to run on
Swain. This keystone rune provides additional power in the later stages of a game. As soon as
Conqueror is fully charged, Swain deals additional damage and gains life steal. This is usually followed up by taking
Presence of Mind,
Legend: Tenacity and
Last Stand.
Presence of Mind helps Swain with the mana problems by restoring missing mana, if a champion is killed. Swain gains extra Tenacity with every additional stage of
Legend: Tenacity. With
Last Stand you deal more damage while you are at low health.
Secondary path on Swain
When it comes to your secondary path, most
Swain players prefer to go with
Domination.
Domination improves burst damage and assassination tools on Swain. On the
Domination path, the best runes for Swain are
Ravenous Hunter and
Cheap Shot.
Ravenous Hunter is the final rune of the
Domination path. This rune gives some survivability during short fights and damage trades during the laning phase. The additional health is nothing to scoff at and can mean the difference between winning or losing a battle.
Cheap Shot makes you deal more damage to enemies with impaired movement, which has great synergy with the setup of Swain.