This build is about how to get extra AP from CDR.
I will try to explain the concept behind this build.
Kass is always a late game monster no matter what, the problem is, sometimes he is so squishy.
So how do you play like a ****ing Rammstein tank and still do a lot of damage?
All the items have armor or magic resist, and the most important things, CDR and MANA!!!
With archangel passive, you get extra AP based on your mana. Normally, kass will buy Rod of ages to get the mana, health and AP.
But you know what, iceborn, frozen heart and archangel gives Armor, CDR and MANA.
You have a bunch of Armor and MR with 2-3 items only, that's when AP kass starts popping off normally.
BUT! You dont just have damage, you are tanky af too.
With around 200 armor or more, 150 magic resist , and around 300 AP.
You will kill everyone when no one can kill you

Act like a *****, let the lane push to your tower and farm.
Just poke with your Q, or use your Q to last hut minions if you can't auto.
Just DO NOT PUSH THE LANE.
Don't try to trade, any top laner will melt you if you don't have your ult to escape.
Keep farming until 6.
If you jungler comes to gank top, try to see if that is a good trade.
You might be able to get an assist or too with your exhaust.

You should be able to 1v1 now.
Try to ult in, do a combo, then ult away.
Your passive can help you because you can walk through minions.
Watch out those champs with hook tho, like Darius.
Save your ult after Darius use his E, or Q silence him then Ult away right away.
Get some towers, dragons.

After 20 mins, you can randomly join a team fight, kill everyone and walk away.
GGWP