Circe loads the ship for the underworld with provisions and these two animals. A black ram, a white ewe. The pair is part of the spell of sorts she makes Odysseus memorise on the beach: the route, the steps to open the way, what must and must not be done. At the dead world’s edge he digs the offering pit himself in the endless mud and pours the libations in their order. Then the two sheep. He cuts their throats and lets the steaming blood fill the shallow hole. That is what calls them. The shades come up out of the mud at the smell of the warm.
The Black Ram and White Ewe
Circe's sacrificial sheep, the two animals you must bring to call the dead. Their throats slit at the trench. Steaming blood fills the hole.