Give Executioner's Calling and Mortal Reminder a restriction
Similar to how Morellonomicon works.
That's it.
As league stands, healer supports are basically non-existent in the meta outside of Alistar whose heal is not even that big anyway (on allies). This will push Sona, Soraka, Nami and Taric (gems) even further down.
My suggestion: Grievous Wounds has two phases, the effect is applied at 100% HP for on-hit healing effects (life steal, spell vamp...) and at 40% HP (currently the Morello threshold) from other sources.
This would not only make the healing supports more viable as it would also be a slight buff for them.
