Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This build is for those who'd like to play a character that can never be killed, and that can be anywhere, anytime.
Let's make Kassadin unkillable.
Start off with some easy 2 pots+sapphire, then head for a classic tear+boots.
Then is the time to choose between a Mejai OR a direct archangel staff. Key here is to pump up your mana pool to function in synergy with your ever increasing cooldown reduction through the game.
Then, head for Nashor's. Added to the runes, this will enable you to pop a riftwalk every 3 seconds or so, making you VERY mobile.
And when you're a very mobile glass canon, you're a threat.