Cosmic Egg is a Keepsake obtained by giving the entity Chaos a gift of Nectar prior to accepting their Boon within a Chaos Gate. Its effects can be enhanced by clearing enough encounters with the keepsake equipped. It is also used in a Minor Prophecy in which Zagreus must defeat the final boss with it equipped.
From Primordial Chaos; you share an Infinite Bond
Even the Origin of All Things seeks to know whimsy and surprise.