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What is the best way to use charm monster? Is it possible to make a monster your thrall with this spell? Should using it multiple times on the same creature have better effects?
A creature under the effects of a charm monster spell is not your thrall, but in some circumstances can be convinced to fight for you (in other circumstances, it may just decide to help you out by eating your allies). Of course, it is probably going to be pissed off at being taken advantage of once the effects of the spell wear off.
Multiple castings of charm monster spells do not have cumulative effects, but it causes their durations to overlap (i.e. If your first spell wears off, your second spell will still be in effect).