[Now, you must understand that you could neither say "no" nor otherwise refuse or fail to do as requested or ordered (short of being completely incapable of completing the action.) Mental Thralldom is just that; regardless of how you'd feel before and after doing something, you have become a slave to the will of the caster, without conscious knowledge. Like necromancy it perverts the laws of nature to come to an ends. All living organisms are afterall free-willed by nature and could not be directly influenced, but here we are stealing away one's freedoms, whether the caster does so on the basis of personal gain or as a means justified by an end. The biggest catch is that the caster himself is no more omniscient or wise than they would naturally be, and is thus every bit as capable of getting themselves into trouble. If I am to be an idiot and unwittingly sign a contract with whatever powers may be without any forethought or previous knowledge of the subject, then I get my way regardless of future consequences. If I didn't request you provide any further insight or forwarning, you can willfully neglect to do so, but your mind becomes incapable of comprehending failure to complete what was requested of you. Truthfully saying "I am unable to do so" is possible, but must be equally sincere. If you're at all capable of getting me into the circle of necromancy, you must do so.
You could respond to "fly across this chasm like superman" with "but I can't fly!" but could not respond to "Jump off a bridge" with "fuck off" or otherwise walk away to do anything BUT jump off the first bridge you come across or know about. Regardless of what occurs between then and now, you're going to jump off a bridge as soon as you get the chance, or likewise run to the first necromantic tome you can find or know about and instruct the caster in it's use at once. There is some imposing of willpower and telepathic introduction of exactly what the caster desires, as well, and telepathy or other unspoken command is possible.
Of course the powers that be, all of them, are thoroughly pissed at any mortal capable of bending the will of others to their own agenda. If I told some god's highest priests to burn all of their deity's books and forsake his name or insulted them by asking worshippers to sin against them I'd be smited right quick. To enslave others is a travesty enough as it is, and the only way to make a good impression in the afterlife is to use it wisely.]