Kassadin is a very good mid laner this season, and is used a lot by many pro players. Kass scales very well and has a great split push potential.

9 magic pen reds
9 mr blues
9 armor yellows
3 ap quints

For Masteries, it is 18 0 12, with the keystone being death fire touch.

Start
Dark Seal- Adds Health sustain in lane, and gives ap and mana. Allows to poke more with q, and get e more often in lane.
Refillable Potion- Health sustain in lane.
SOMETIMES Boots- Against champs that have skill shots to dodge or can poke down a lot in lane. Champs like Ziggs or Arhi are good examples. This would be in for the dark seal, and get an extra Health pot

Full Build in order
Rod of Ages- Gives health, mana, and ap. Passive is great for scaleings. Other passive gives more sustain for more uses of ultie.
Lich Bane- Gives ap, movement speed, cdr, and mana. Very good for more ap, and more ad due to the passive.
Zhonyas's Hourglass- Gives ap, armor, and cdr. Active is very good for late game, as it is more devistaiting to die late game.
Void Staff- Gives ap and magic pen. Just to add on the damage late game.
Rabadon's Deathcap- Gives ap, and does what void staff does, add on more damage late game.

Boots
Mercury Treads- VS Heavy AP
Ninja Tabi- VS Heavy AD
Sorcerer's Shoes- Very game dependent, most likely better to go Mercs or Tabis.

Max q first, e second, w third, and your ultimate when ever you can.

Flash and Teleport

-Pros
Very good late game, and is very good at split pushing.
Very hard to gank past level 6.
Scales very well.
Great mana sustain in lane.
Insane escapes and chases.

-Cons
Can be zoned from CS.
Easy to kite sometimes.
Not the best early game.

Use your w to sustain mana in lane. Do not die. Play passively until level 6, for the escapes. Never trade unless it is beneficial. After laning phase, split push a lane.

Type /mute all in chat every game. It will make you play better, and not have to worry about toxic players. Communicate with pings.

This is my first ever guide on Moba Fire. I hope you enjoy the guide, and if there is something wrong, send your feedback. :)