Flying
When this creature enters, mill three cards. (Put the top three cards of your library into your graveyard.)
Threshold — This creature gets +2/+1 as long as there are seven or more cards in your graveyard.
Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to Billowing Shriekmass may become lethal if the number of cards in your graveyard falls below seven.