Michael: And to bind the spell together, the nectar from a Leviathan Blossom.
Dean: A Leviathan Blossom? What is that, like a flower?Michael: A flower that only grows in one place. Purgatory.
A Leviathan Blossom is a special flower that grows exclusively in Purgatory. It sprouts from the rotting remains of a dead Leviathan but can take months to grow. It's described as having ten husks and bloody red centers. 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.