What?!? Yes, it’s all true. No matter what you've been told, or read, the ‘katabasis’ of in Odyssey 11 is not an actual katabasis!
A katabasis is a descent. In this context it’s a descent into the Underworld, but it doesn't necessarily have to be. And there are a bunch of famous katabases that we all know and love: Herakles going down to steal Kerberos, Orpheus heading down to rescue Eurydike, Theseus and Perithous’s attempt on Persephone. But, Odysseus isn't one of them. Katabasis stories involve physical travel into the Underworld, not realistic religious rituals. And the dead are not used as intermediaries. Odysseus's so-called 'katabasis' does involve a religious ritual and he interacts with the dead.
Odysseus starts in the living world, he digs a pit in the earth and pours his offerings into it. The dead rush up to greet him. This all shows that Odysseus is performing a necromantic rite, but there is a little bit more. So, let’s have a look at the actual necromancy itself (Homer’s Odyssey 11.20-47). This translation is from the Chicago Homer, where you can also find the original Greek.
We beached our ship when we got there, unloaded 20
the sheep, and went back ourselves along Ocean's stream
until we reached the place Circe had described.
“There Eurylochus and Perimedes held the sacred victims,
and I drew my sharp sword from beside my thigh,
dug a pit a cubit's length this way and that, 25
and poured a libation to all the dead about it,
first with milk and honey, thereafter with sweet wine,
a third time with water, then sprinkled white barley groats upon it.
I repeatedly entreated the helpless heads of the dead,
that when I got to Ithaca I'd offer a cow that's not yet calved, 30
my best one, in my palace, then I'd fill the pyre with good things,
and that I'd sacrifice separately, to Teiresias alone,
a solid-black ram, that stands out among our sheep.
After I'd implored with prayers and vows the tribes of corpses,
I took the sheep and cut their throats 35
and the cloud-dark blood flowed into the pit. Up out of Erebus
they gathered, the souls of the dead who'd died,
brides, young men never married, old men who'd suffered much,
tender maidens with hearts new to sorrow,
and many wounded by bronze spears, 40
men killed in battle, holding armour stained with gore.
They stalked about the pit in throngs from one place and another
with an awful screeching, and green terror seized me.
Then at that moment I urged and ordered my comrades
to skin and burn the sheep that lay there slaughtered 45
by ruthless bronze, and to pray to the gods,
to mighty Hades and dread Persephone.
Offerings to the Olympic gods were (usually) upward focused. Sacrifices were burnt with the smoke rising up, and prayer conducted with arms raised into the sky. It makes sense that it happened this way – the gods are up, so offerings need to go up. So too with Underworld gods, except they are down and the offerings need to go down. In this section of the Odyssey, Odysseus digs a pit with his sword, and pours his offerings into it – including the blood of his sacrifice – and they sink down through the earth to the Underworld gods. The same thing happens in historical supplication to Hittite Underworld gods, where there is evidence for temporary, downward facing altars made by digging a pit in the earth. Sometimes this happened on a riverbank, and there’s evidence that pits might have been dug out with daggers (see Collins 2002, linked above). While there’s a strong link here, there’s a stronger link to Greek necromantic rites.
Odysseus is told, by Kirke, to go and find answers about his journey home from a dead seer, Tiresias. The place she sends him to matches the location of the Oracle of the Dead near the Acheron, in Thesprotia. Pausanias, much later of course, tells us that Homer knew that he was writing a description of the site at Ephyra (Od. 10.508-514):
But when you drive through Ocean with your ship,
there will be a rough headland and groves of Persephone,
tall poplars and willows losing their fruit. 510
Land your ship at that spot, by deep-eddying Ocean,
but go yourself to the dank house of Hades.
There Pyriplegethus and Cocytus, which is a branch
of the water of the Styx, flow into Acheron
Much later Pausanias (1.17.5) says that Homer had been to the Nekyomanteia here and used it in his description:
I think Homer had seen these places and boldly ventured to describe Hades’s realm in his poem and further named the rivers after those in Thesprotia.
Of course, ‘Homer’ himself doesn't say anything on the subject, but the topographical and archaeological evidence suggests that where Odysseus goes is indeed the location of the historical Oracle of the Dead. Homer could have included this because there was a cult of Hades close by (but that’s a story for another time).
Although this is only a short exposition of the issue, the two main points certainly illustrate that Odysseus doesn't descend into the Underworld. He isn't described as being in the Underworld, but the dead are described as rushing up, out of the Underworld, to him. And the site described by Homer is the site of an Oracle of the Dead.