We don't call it a flower. There's a blossom. Grows out of the soil when we die.
It is described as having a tan husk and a bloody red center. Combined with myrrh, cassia, and rock rose, its nectar is one of the necessary ingredients for a spell designed to bind God, just as He and the archangels bound the Darkness.
After learning the truth about Chuck from Castiel, Michael gives him and Dean a spell with a list of ingredients that can bind God in His weakened state. One of the ingredients is the nectar of a Leviathan Blossom, which grows exclusively in Purgatory. To retrieve the flower, Michael opens a rift to Purgatory for Dean and Castiel that will last for 12 hours.