I think we should have a troop that has the ability to cast mind control. The troop can control the enemy troop and could attack by using spells or by physically do damage to the others health by attacking them. The only way to stop the effect would be by defeating the troop that inflicted mind control. If all enemy troops was under the spell the game would be over unless they snapped out of the trance.
Either you include the mana/skull requirements, which makes the effect not very useful. Or you don’t, and it becomes extremely overpowered.
The only middle ground, I could think of, would be, that it is not a lasting effect, but perishes after use.
P.S.: We do have sort of that thing in the game already with the charm effect.
You might consider adding a magic counterpart, but on the other hand, the majority of spells are not even damage based, and “turning” each of them “around” could lead to some very confusing effects.
P.P.S.: If you want to follow that thought of mana-charming, you end up on a viciously wide scale, that ranges from incredibly powerful (enemy Zul Goth decimates its team at your command) to complete backfire (Black Beast devours your troops).
And of course, there will be bugs. Loads of bugs.
Charm, yes; also the magic-stealing Ilithia faction troops have kind of a mind-control theme.