Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Swain - Kick Starter
This is how i start Swain. the Doran Rings give great early gank hp, ap, and regen. After Dorans I recommend using a build site because I customize my build for the enemy team (and dont wanna spend all day on this lol). that goes for the runes too, but I recommend the penetration runes for early ganking.
I start tryin to gank at level 3-4. I open with a Never Move on the lowest hp enemy, then if it hits I combo Torment + Decrepify + Ignite and stay in range to auto attack.
Note: try to bait the enemies flash, cleanse, and ghost before using ignite in the full combo.
This combo does a ton of dammage early and even later games can take half the hp bar of a lower level enemy or squishy when combined with Swains ultimate.