Put a +1/+1 counter on target creature you control if it's legendary. Then it fights target creature an opponent controls. (Each deals damage equal to its power to the other.)
4/27/2018 You can’t cast Ancient Animus unless you choose both a creature you control and a creature you don’t control as targets.
4/27/2018 The creature you control doesn’t have to be legendary. It simply won’t receive a +1/+1 counter before it fights if it isn’t legendary.
4/27/2018 If either target is an illegal target as Ancient Animus tries to resolve, neither creature will deal or be dealt damage.
4/27/2018 If the creature you control is an illegal target as Ancient Animus tries to resolve, you won’t put a +1/+1 counter on it. If that creature is a legal target but the creature you don’t control isn’t, you’ll still put the counter on the creature you control if it’s legendary.