They could agree on one thing only: one of them must die.

Rivals' Duel

Choose two target creatures that share no creature types. Those creatures fight each other. (Each deals damage equal to its power to the other.)

The List

Planechase Anthology

Planechase 2012

Morningtide

Choose two target creatures that share no creature types. Each of those creatures deals damage equal to its power to the other.

If either one of the creatures leaves the battlefield before Rivals’ Duel resolves, no damage is dealt to or by the remaining creature. If both creatures leave the battlefield before Rivals’ Duel resolves, the spell doesn’t resolve for having no legal targets.

If, by the time Rivals’ Duel resolves, an effect has caused the two target creatures to share a creature type, Rivals’ Duel doesn’t resolve for having no legal targets.

The two creatures may be controlled by the same player.