(sorry if it seems like i'm spamming, i've been playing on other accounts since 2012 and I have soooo many ideas)
The spell would be: Purloiner (Can't think of a better name)
Basically, it's a trap that you place on an enemy, when someone heals the target with the trap, the caster of the trap will intercept 10 - 25% of the healing done (?)
It could be disabled in pvp, but if it wasn't it could be a good counter for the Guardian Angel spell, it's also handy if your low on health and pips.
The only use for a spell like this would be in pvp but I don't think it would get much use in this form. It would work better if it took 25% from all heals cast and worked as an aura or a shadow spell like Sentinel.