Mind Control is a fourth eraNature spell that permamently bolsters your forces while denying the enemy of theirs.
As it works against almost all non-boss units when upgraded it is a good way of gaining access to creatures not in your deck in a late pve game or indeed in anybodies deck when used in pvp. It should be noted that effects such as paralysis will remain on the mind controlled unit.
While the spell initially costs more power than the power of the creature obtained (50 to 130 more, depending on how high Mind Control is upgraded), there is no power lost permanently, and there is a net gain as well. Normally, the power spent on a creature only returns to the power pool when it dies. When taking over a creature with Mind Control, however, the Power bound by that creature is immediately returned to the original owner's void and the power used by mind control returns to your void as well. As a result, the controlled creature no longer binds any power from either player and returns no power to the void when it dies. This can be a significant advantage for the mind control user, inasmuch as they get to control a creature but have no void power bound up in it as is normal.