SergOz,
there are several ways of doing this. Daemons proposal is difficult because you need many checks if prayer is active and who casted it.

In ACM I perfected this system. Here is the idea in principle:


*you can either reduce the power of prayer by the same amount you increased it, or you save the power on map start and always revert to that start value. To avoid conflict I recommend the first approach.

Now if you want the effect to also show in the spellbook the script gets much bigger. You need to set new descriptions for the spell with every turn in combat to adjust to the new powers but it looks much cooler.

Also in this case I would not use UN:C. ERM has all commands needed.