The power and effect of spell is given in the bracket [x] Ranging from 0 to 104. The spell can be cast multiple times to try to improve the effect of it. Success of casting the spell depends on your channeling skill. A higher power increases how keen (fast) the Messengers are to work for you. In other words, mail is delivered faster if the mailbox has a higher cast on it. The amount of time this takes is 101 minutes - enchant power.
Dark messenger can also be cast on fountains and wells to turn them into wishing wells, which can then be used to make wishes by activating a coin and right-clicking the fountain or well. It is unknown whether the cast power affects the chance of the wish to come true.
Failure to cast uses 1.5 favor.