I have just found an issue, that seems to be bug. I have cast a burning crusade on my Angel of Mercy, then used the grant: magical aura from Northern Cross Priestess. Opposing champion was on 1-5 hp (I have tried multiple times), and at the beginning of opponents turn, opposing champion got damaged by auras, then mercifull triggered and the opposing champion was set to 1hp. Mercifull clearly states, that mercifull triggers when an opposing champion should be killed by an attack from champion with mercifull. Since aura isnt attack, either the ability or its decription should be changed.
It is probably that the description for Merciful needs to be updated, but we will look into this, thanks.
If an enemy unit is at 5 or less hp and the unit with merciful kills the opponents warbanner the opponents unit loses 5 hp and then has its health set to 1.