Swain
Another Opponent, Another Disappointment
Lane harassment and consistent damage output.
Sorcery
Empowered abilities and resource manipulation
Phase RushHitting an enemy champion with 3 separate attacks or abilities grants a burst of
MS and Slow Resist.
Nimbus CloakAfter casting a Summoner Spell, gain a short
Move Speed increase that allows you to pass through units.
CelerityAll
Move Speed bonuses are 7% more effective on you and gain1% Move Speed .
ScorchYour first damaging ability hit every 10s burns champions.
Inspiration
Creative tools and rule bending
Triple TonicUpon reaching level 3, gain an Elixir of Avarice.
Upon reaching level 6, gain an Elixir of Force.
Upon reaching level 9, gain an Elixir of Skill.
Cosmic Insight+
18 Summoner Spell Haste
+10 Item Haste
Adaptive Force
+9
Adaptive Force Adaptive Force
+9
Adaptive Force 
Health Scaling
+10-180 Health (based on level)
Why Phase Rush
Phase Rush on Swain massively empowers your ability to stick to your opponent while casting your ult as well as being able to stay in range for follow-up [Q] damage.
Why Nimbus Cloak
Nimbus Cloak's massive burst of movement speed is great for chasing down enemies and getting to the back-line for a large explosion.
Why Celerity
The bonus Movement Speed from Celerity is pivotal in re-positioning for landing a clutch [E] root and maneuvering with your [R] active in teamfights.
Why Scorch
Scorch is an incredibly strong early laning rune for pressure and harassment. Look to proc the bonus damage as often as possible with either [Q] or [E].
WhyTriple Tonic
Zhonya's and the stasis are a key part in taking over teamfights as Swain thanks to the ability to keep your [R] active and healing you throughout.
WhyCosmic Insight
Cosmic Insight is an all-around fantastic rune choice for any champion as the extra bit of CDR helps you pump out more damage and stay safe with increased Summoner CDR.