Swain is tanky control mage. His point is to just wade into the middle of a team fight with his R along with rylais and just slow/sap health from everyone. With a build along the lines of ROA/Rylais/Zhonya's/Liandries and maybe, if you're doing extremely good, a death cap for extra damage, if not, usually a defensive item. If you use him correctly he can do a lot of damage and provide a decent amount of CC when he lands nevermove on multiple targets due to rylais slow.
Right now he is in sort of a weak spot due to Riot trying to speed games up. He needs 2-3 items to do his thing and usually by that time the game is already snowballed to one side or the other. But if he gets a chance to do his thing he can put out lots of damage. Like anivia/viegar he needs time to scale up into being scary. He doesn't start scary.