The idea is, that Kassadin's R does scaling damage when you use it more often and it scales on his maximum mana. Its cooldown is very low at lvl. 16 and the burst from it is usefull for teamfights, 1o1's, engaging and escaping. The new midseason update changed some items in a real annoying way. So I though why not search for the advantages of this? Kassadin is the best way to present them.
Morello now giving maximum mana and mana on kills and assist fits perfect with a kassadin ultimate.
Hextech GLP-800 has a low-cooldown active slowing enemies, gives you mana and ap and even deals damage.
Seraph's shield is good for hard engages with a shield about 500 damage at fullbuild and the ap you get from it is insane.

You won't get ap early in game. If your matchup is even or weak use that advantage and call for some ganks. If you win the early you will be able to totally destroy the lategame.
Try to play safe otherwise, concentrate on farming, deny enemy trades with your spellshield, if you have to, farm with abilities. This build is not recommended vs. strong enemy early junglers.

Your strenght begins with Hextech GLP-800. This active item allows you to chase enemies and burst them down. Due to the intense mana you get from this build, your ultimate gains damage, as well as you can use it quite often. Its all about trusting your scaling in midgame.

Buy Rabadon's. Kassadin's ap will be about 933 (with my runes) and still you got Seraph's shield you can spam your R because of the Morello passive giving you mana back for kills and assists.