My suggestion is to add another castable option for spell called death and decay. To use it as paladins spell called consecration. It would be option, to cast it as original or as paladin does. To solve this as easy as it can be. I would suggest to make two versions of spell, one which has original castable properties and one which has one click cast as consecration under your feet. Both should have shared coldown, to prevent casting of both in same time.
Why? As tank, it is quite annoying to always cast this spell around you with two clicks, in 99% of situations, you need death and decay under your feet anyway. Paladin tanks has easier life, they just click it and it is under their feet.
