Anti-heal Cantrip

Your onto something but I'd suggest that you raise the spell level because what it sounds like is that the user is drawing forth a 15 foot radius of the negative plane now this is cool but if this is the case than that is some serious magical power anyone in that zone would probably also have to make con saves and take idk let's say 3d8 necrotic damage every time they start in that zone remember the negative plane is where undead are drawn forth from or at least the energy user to create them the negative and positive are in a consistent push and pull and the negative is a zone of what can only be described as absolute death anyone who travels there needs to really prepare otherwise the plane will suck the life force out them and turn them into a special type of undead native to the plane a sort of spector of negative energy these are the same beings that bring undead in the prime into unlife and dun fact in the negative plane healing works exactly how its described to work in ur spell 

Another thing you could do is instead turn this into a spell that heals undead by drawing energy from the negative plane instead of the positive let's say it heals an undead for 1d4 but when used on a living creature it instead deal that damage this way u can keep it a cantrip and maybe rework the other idea into something of a higher level that u could pull out to both heal ur undead and do extra passive damage to ur enemies