Healing used to be in the perview of the necromancy school. Also, necromancy is playing with both the body and the soul. A necromancer channeling life energy/forcing it to flourish in a target, or literally forcing their wounds to close (in a sort of stopping blood from flowing by forcing it to clot) in a sort of biomancy kind of way would be interesting. How does this strike you:Well, we know the Assassin is a Striker.
Unless they're fooling us, the Hexblade will probably be a Defender. I find Leader unlikely, but possible.
That leaves the Necromancer as a Controller or a Leader. I think the former is much more likely; I don't see Necromancers getting any kind of Encounter healing ability like every other Leader does.
Rotting Word
You channel necrotic energy and force shed blood from the battlefield back into the target, and seal their wounds with dead skin. An aura of decaying magic lingers on the target.
Minor Action (Close burst 5)
Special: You can use this twice per encounter, but once per round.
Target: You or one ally.
Effect: Target can spend a healing surge, and until the end of your next turn, the target gains necrotic resistance and deals necrotic damage equal to your wisdom bonus.
An alternative would be the target gaining temp HP, represented by dead flesh providing some means of softening it up.