Hey There! Midlane is getting a bit boring? Well you should absolutely try this build on Kassadin. It's a really, really late game, but you should be able to kill someone each 3 seconds at minute 30. Before that,.... well farm.

Everything is based on The Dark Harvest! Each time you get a kill you auto will deal a lot more damages, enough to kill an adc with an R and a W at late game.
As second, I pick Sorcery with "The ultimate Hat", to get a 15% cdr on the ult. Might be a little overkill, but having his ult every 1.02 sec makes you hard to catch... or escape from you.

This build is based on :Mana, then Lichbane.

When you get the lichbane, you can start to oneshot, should you not be too late in the game

+ Late game one punch man
+ infinite escape
+ same burst damage each 3 sec
+ game won at 30min

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- Early nonexistant
- needs a lot of time
- literally useless before 2-3 core items

You're at 30 minutes or more, and the team is ready to fight with you? well that's perfect, because you should be able to OS anyone that's not tanky, and that usually means 2 or 3 opponents.
You have a huge impact because the ennemy team can't ignore you, and you can TP every 1 sec. Meaning you can literally jump in, kill someone and get out, then repeat for as much time as you want. Your team is merely there to assure you won't get focused, or that the target stays reachable.