How do I calculate the difference in cost of the two minions?
Start with the cost of the Evilminion in your hand, then subtract the cost of the Mortal at the Pond. Remember that costs cannot be negative, so if the result is belowzero, treat it as zero instead.
Can I transform a Mortal I do not control into Evil from my hand?
Yes, though since it is a transform effect, control of the transformed minion will not change. The player who controlled the Mortal will control the Evil that came from your hand.
