Decay Question. (in General)

Hello all, Im sorry if this question has be answered ad nauseum in cb1.

Why would one raise the level of the Decay spell since it always takes 1/2 of the opponents hp away?

IE, is there a chance it will not work at the base level, and that chance goes up as the spell level goes up? If so, anyone know or have an idea of the factors that go into determining whether or not it works?

Thanks for your time.

Resistance against AMF.

AMF reduces DD damage based on it's level versus the DD s level. A base Decay gets reduced to nothing from high AMF s.

You raise the level of decay to protect against AMF. The higher the level the less effect AMF has on your decay.