Of most of the monsters which were killed by various heroes, their shades or spirits were placed at the gate of the Underworld by its king Haides (Hades) to help Kerberos (Cerberus) the three-headed watchdog of the realm to guard this gate. These dead monsters were Kerberos’ brothers the two-headed hellhound Orthos, and the triple-bodied monster called the Khimaira (Chimaera), as well as the Sphinx, the Nemeian Lion, the Kithaironian Lion, the Caucasian Eagle (or hawk or vulture), the Krommyonian Sow, the Hydra of Lerna and the dragon Ladon.
One might say that all the deities of the Underworld were spirits, including its king Haides (Hades) himself, and its queen, Haides niece and wife Persephone. Next to them the most famous spirit of the realm was Kharon, the rugged old boatman who ferried the shades of dead human beings to the court of Haides where they were judged.
Hermes Khthonios (Underworld Hermes), who conducted the shades of the dead into the Underworld; the night-goddess Nyx and her sons Hypnos (Sleep) and Thanatos (Death); the three Moirai, the goddesses of fate and destiny; Hekate, the Titan goddess of witchcraft; and the ancient Titans who were overthrown by Zeus, among whom was Zeus’ father Kronos, were all thought to dwell in the Underworld. Erebos, the personification of primeval darkness, filled the entire realm of Haides with himself.
The Keres, a coven of goddesses who personified violent death and demise through disease, daughters of Nyx and sisters of Hypnos and Thanatos, also lived in the Underworld. The 1,000 Oneiroi (Dreams), sons of Hypnos and Nyx, lived there also, as did Epiales, a nightmare-demon who may have been related to the Oneiroi or was one of them himself. Eurynomos was a demon of the Underworld who stripped corpses of their flesh, and perhaps was also related to Nyx’s children if he wasn’t one of them himself.
Monstrous demonesses called the Empousai (Empusae) were the daughters and attendants of Hekate. Underworld nymphs called the Lampades, who carried flaming torches wherever they went, also accompanied Hekate together with a pack of hellhounds from the River Styx. I imagine these dogs weren’t too much different from Kerberos and Orthos, and perhaps had similar origins. The Arai, evil spirits who personified curses, also dwelt in the Underworld, from which witches, the servants of Hekate, would summon them when they wished to curse a person. Vampire-like demonesses called the Lamiai, and another one called Mormolykeia, were also associated with Hekate and the land of the dead.
The Underworld had five main rivers, among which the Styx was personified as a goddess, and the Akheron and Kokytos were personified as gods. Askalaphos, who used to be Haides’ gardener, was the son of Akheron. Askalaphos’ mother, called Orphne or Gorgyra, was a nymph of this realm. Minthe, a lover of Haides who was transformed into the mint plant, was a Naiad (river-nymph) and daughter of Kokytos.
Haides’ herdsman Menoites and his father Keuthynomos were spirits of the Underworld.
Leuke or Leukippe and Daeira were Oceanides of the Underworld.
Makaria, the goddess of the blessed dead, was the daughter of Haides and Persephone.
The demoness Melinœ, daughter of Zeus and Persephone, was a creature of the Underworld.
Tartaros was the personification of the deepest part of the Underworld, the Abyss in which the Titans and all sinners were imprisoned. The River Tartaros which flowed in this deep chasm was also personified as a god.
The spirits of the prophets Amphiaraos and Trophonios had oracles respectively at Oropous and Lebadeia, both in Boiotia. The spirits themselves were believed to dwell in the Underworld.
The three judges of the dead, Aiakos, Minos and Rhadamanthys, all dead sons of Zeus, were spirits of the Underworld.