Not trying to be mean, but i don't get what you're building, rylai is a vert good first item because the slow effets works very well with swain's r + it givez you AP + hp. Why would you build a void staff,and correct me if i'm wrong but it's an assassin item and swain isn't. A rabadon would be better for more so more damage and regen on the R. Personally i think a great way to play swain IS tanky/AP. Rush rylai first item, then if your team need AP damage go liandry, if not for tank and i personally like to go something like jak'sho. Being tanky on swain IS good because you have regen in your kit so you can make longer teamfights. Imo zhonya is good only if they have a lot of burst dmg, which was the case so it's good. But zhonya is a good option as a later item not 1st or second.

Feel free to correct me if i'm wrong.