At the beginning of your upkeep, if Skulking Sphinx is in your graveyard, each opponent chooses 'sleep' or 'struggle'.
At the beginning of your upkeep, if Skulking Sphinx is in your graveyard, you may discard three cards to return Skulking Sphinx to the battlefield. If you do, each opponent that chose 'sleep' sacrifices a creature. If you do not, each opponent that chose 'struggle' sacrifices a creature.
See Challenge # 115.