The Hera's Lily is only available as an exclusive item in the Hera Pack. It allows you to choose the coat of one or more foals from among the coats allowed for their breed.
Hera's Lily is a one use only item, like other breeding items. You must give your mare the Hera Pack before the covering takes place in order for it to count for that breeding. Once your mare has given birth, you will be able to select the color of both foals. Note: Both foals will have to be the same color - You cannot have nonidentical twins.