Why is Heracles in the Underworld?
Odysseys during his travel to the Underworld in Book XI of the Odyssey meets the "phantom" of the deified Heracles:
And after him I marked the mighty Heracles—his phantom; for he himself among the immortal gods takes his joy in the feast, and has to wife Hebe, of the fair ankles, daughter of great Zeus and of Here, of the golden sandals.
Why are there two versions of Heracles in the afterlife? How is the phantom version Odysseys meets different from the one enjoying himself in Olympus?
Heracles was a demigod. After his 3rd wife thought she had an adversary, she pulled out the vile of Centaur blood she received years before and spread it on his shirt. The Centaur told her that his blood was a kind of love potion, but it was actually a kind of acid. So, when he put the shirt on, he grew in pain. His skin melted and the shirt melded to his body, inabling him to take it off. The pain was so intense, he built a pyre to die. But, it was long decided by Zeus that Heracles would become a god upon death.
So, Heracles the demigod became a full god. Because Odysseus saw him in the Underworld, we know that his mortal self made its way to the Underworld like all those who die.
What was left of Heracles's mortal self traveled to the land of the dead. At the same time, Heracles's godly self is up on Mount Olympus.
This is all very interesting, but is it supported by ancient texts? Did the Greeks differentiate between "mortal self" and "godly self", and was that reflected in their mythology?