I would say that the immortal/immaterial status itself is OP,(and especially the ability to put it on any unit, over and over for a affordable price) there is only one way to kill em, the life poison shield (which takes time) and no way to deal with their immortal status. Plus to counter that, one can destroy or steal the shield without much effort/cost. Yet the owner can target their own immortal creatures to trigger their abilities. Its even worse that immortal even protects the creature from field effects that don't specifically target them.
So if this spell had the ability to remove immortal status with enough saved death energy, I would say it would be just the minor nerf immaterial/immortal status needs, rather then being overpowered itself. (I would say it needs other nerfs too, like no card should be able to grant it as a ability, or at least not for anything less then a large amount of quantum[10? 20?])
Actually, the whole slay idea here was in part a inspiration along the way of thinking up ways of nerfing other OP stuff like immortal status a bit.
Remember, its either remove immortal status (if enough death saved up), OR do damage if no immortal status, and it being a single target spell and not a repeatable ability, this would be limited. (plus one can make a creature immortal again if striped of it but not killed)