Concept: How to make a Tank/Assassin
It's the one combination that seems truly impossible for League to create. An Assassin Tank. But I think I just cracked the code.
Assassins don't necessarily have to do damage quickly. What they have to do is assure the kill.
So here's what I'm thinking. An ultimate that does a severe damage over time effect, or worse yet, gradually reduces your maximum health as you waste away (no hope of healing it). If not stopped in time, it will undoubtably kill the target, with the counterplay being to kill the tank/assassin guy before his curse finishes you off.
Obviously, there remains quite a few problems with the idea (most notably, what if he just casts it and runs in the opposite direction?), but I feel like it's a good basis for a design to jump off of.