Who is Odysseus trying to summon by making sacrifices in Hades?

When Odysseus and his men sail to the underworld, they are
trying to summon and meet the ghost of the prophet Tiresias.  From him, they are going
to try to get advice about how to get back home.  This happens in Book
XI.

Previously, in Book X, Odysseus had gotten advice from
Circe about how to get home.  She told them that Tiresias would know the best way for
them to manage this.  Because she tells them this, they go to the land of the dead and
start to make sacrifices.  The sacrifices that they make attract many, many ghosts but
Odysseus holds them off and will not let them get at the blood.  He only wants Tiresias
for the advice he can give.  Later on, Odysseus meets many other ghosts, including his
own mother.  But the whole point of the sacrifices is to summon
Tiresias.