Mind control is not a particularly powerful Epic Event.

But as with Tacullu’s Mind Capture, it helps both in defense and offense, and is always a lingering threat. 
Early game can be very good to stop and punish enemy rushes.  Midgame it can turn around assassination attempts on Tacullu that involve the opponent's summoners.  Late game when both summoners tend to clash and are not at full life, this is when Mind Control is at its peak, allowing Tacullu to turn around the opponent's army and possibly assassinate the enemy summoner.