Hades is the real deal, yo. He does the Underworld do. King of the Underground, but not the firey pit of hell that most people refer "underground" to.
The Underworld consists of four main parts-
The Fields of Asphodel, the Punishment Fields, the Elysian Fields/Elysium, and Tartarus.
The Fields of Asphodel are mainly what mythology described as the "grey endless fields". These are where souls go when they have neither dishonored the Gods, nor pleased Them. So they just kind of stand there. For eternity.
The Fields of Punishment can be equivalent to what the Norns call "Helhiem", or what Christians call "Hell". It's a really bad place. The Punishment Fields are where people, even Gods or Titans, have offended the Olympians or other Gods. If you all aren't familiar with Sisphyus, this is where he now resides.
Sisphyus was a kind of Greece, who stole the secrets of the Gods, and shared them with the public. His punishment in the afterlife, was pushing up a spherical rock up a hill. The only problem with that is, once he is about to reach the top, the weight of the rock falls back to the bottom of the hill. So Sisphyus has to start over. The rock never reaches the top, it always falls down.
Anywho, onto Elysium. These fields are the Islands of the Blessed. The heros and the people who have well pleased the Gods reside here. This is also equivalent to "Heaven". And in the Greek's minds, it is also called Heaven.
Tartarus is an extremely bad place. This Pit (an alternative name for Tartarus is "Pit") is reserved for those who fought against the Gods. Monsters, Titans, and other Gods reside here.
Pardon my random ramblings, but... Onto the actual beings of the Underworld.
If you were to pass onto the Underworld, you would be accompained by Charon.
Charon is a servant of Hades. He isn't actually a God, but a daimon (spirit). He accompanies those who have passed over the rivers Styx and Acheron(river of pain), onto the Fields. The fee for his service was a single obolos coin which was placed in the mouth of a corpse at burial. Those who had not received due burial and were unable to pay his fee, would be left to wander the Earth, haunting the upper world as ghosts. I personally think that the Fields of Asphodel is the plain where "ghosts" interact with humans on the physical plane. (That's just me, though)
Cerberus was the gigantic hound which guarded the gates of Hades. He was posted to prevent ghosts of the dead from leaving the underworld. Cerberus was described as a three-headed dog with a serpent's tail, a mane of snakes, and a lion's claws. Some say he had fifty heads, though this number might have included the heads of his serpenty mane. The name Cerberus is a Latinised version of the Greek Kerberos, which is related to a Proto-Indo-European word "kerberos", meaning spotted.
Yes, you read that correctly: Hades, the mighty Greek God of the Underworld, had a pet dog namedSpot.
Thanatos (Latin: Thanatus) is the God of non-violent death. There isn't much said about him in mythology itself. There was one story stating where Sisphyus captured Thanatos and placed Him in his pocket, to avoid death. Violent death was the domain of Thanatos' blood-craving sisters, the spirits of slaughter and disease, the Keres.
The Keres were the female spirits (daimons) of violent or cruel death, including death in battle, by accident, murder or ravaging disease. The Keres were cravers of blood and feasted upon it after ripping a soul free from the mortally wounded bodies and sending it on their way to the Underworld. Thousands of Keres haunted the battlefield, fighting amongst themselves like vultures over the dying. Not very happy spirits.
The Erinyes were three netherworld goddesses who avenged crimes against the natural order. They were particularly concerned with homicide, unfilial conduct, crimes against the gods, and perjury. They were the servants of the Fates. The wrath of the Erinyes manifested itself in a number of ways. The most severe of these was the tormenting madness inflicted upon a mother or father. Murderers might suffer illness or disease; and a nation harbouring such a criminal, could suffer death, and with it hunger and disease. The wrath of the Erinyes could only be hostile with the rite ritual purification and the completion of some task assigned for a wronged injury.
Part three will be soon!