Part of it is knowing how to build swain, cause he's hella item dependant. He needs a mana pool, but he also needs to be tanky, so RoA is pretty much a must on him. DCap makes him hit like a truck, Liandry's works well with his high DoT kit, and hourglass will still allow his ult to go. Combined with some flat magic pen, and maybe an abyssal or void staff if necessary, and you're one scary fucking bird.
He's basically a team-fight long fiddle ult with aoe cc, complete with cawcawcawcawcawcawcawcaw.