Mana Vortex

You and target player both exile the top five cards
of your deck. Either player can then tap any
amount of land, and exile any amount of creatures
they control. Then each player counts the
combined mana cost of all cards
exiled, +1 for each mana produced by tapped land.
The player with the lowest number takes damage
equal to the difference between the two numbers,
up to 15. If the difference is above 15 the winning player gains X life, whereas X is amount above 15.