That sounds like something that would not work well in 5E.
First off, full-caster Monk. Overpowered-Monks are great on their own, slapping on 9th level spells is crazy.
Second off, abilities based on monsters slain. It's really hard to work out-I should know. I've tried to make a Megaman class, and it just doesn't work well if you have to kill/defeat stuff to get powers.
So, I ask once again-what's the FLUFF concept? Ignore mechanics. Just tell me what the fluff.