Odysseus summons the spirit of Tiresias by bringing a lamb to sacrifice. Odysseus is summoning the dead in a way called <em>necya</em>. It means that some dead being has to be sacrificed.
In this case, Odysseus cut off the head of the black lab and fill in the pit with its blood. This way the spirits were attracted. He gave the most of the blood to Tiresias as he was the only one who could prophetize after his death.
