Swain is the newest mage to League of Legends. His damage output as well as lane control are extremely strong, and I can see him used as an anti-carry in his career as a champion. This video focuses a lot on his spell combos, which I think is a key skill to learn while playing Swain. The ideal combo to use is Nevermore, Torment, Deathfire Grasp, Decrepify, with Ravenous Flock running the entire time. However, this is not always going to be a feasible combination of spells.
Often, you’ll be fighting enemies at long range and won’t be able to land the spells in the best possible order. Leading with Decrepify to slow your opponents in order to ensure all your spells will land is an acceptable loss in damage output, since Torment just in itself is his single largest damage ability. Similarly, during the laning phase, just using Decrepify Torment will help you conserve mana since Nevermore will be easy to dodge with a low level Decrepify slow. Hopefully you’ll find Swain fun and an excellent addition to your team!