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.
WhyNimbus 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.
WhyCelerity
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.
WhyScorch
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].
WhyPerfect Timing
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.