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.)
You canâ€™t cast Ancient Animus unless you choose both a creature you control and a creature you donâ€™t control as targets.
The creature you control doesnâ€™t have to be legendary. It simply wonâ€™t receive a +1/+1 counter before it fights if it isnâ€™t legendary.
If the creature you control is an illegal target as Ancient Animus tries to resolve, you wonâ€™t put a +1/+1 counter on it. If that creature is a legal target but the creature you donâ€™t control isnâ€™t, youâ€™ll still put the counter on the creature you control if itâ€™s legendary.
If either target is an illegal target as Ancient Animus tries to resolve, neither creature will deal or be dealt damage.