River Styx: Also known as the Pit of Death, this river carries the dead to the Underworld, where they spend the rest of eternity floating in its waters. In the episode "Hercules and the River Styx" of the animated series, it is revealed that the river is also a Goddess that only reveals herself to those who wield Poseidon's Trident. It is also revealed that the river marks the border of Hades' domain. Technically, Styx is in fact a nymph and will live for as long as her river flows.
Tartarus: is the deepest, darkest, and most sinister area of the Underworld. It is here where the souls of the wicked and evil are sent and it is also the place where Zeus imprisoned most of the Titans and a giant Cyclops after he ended their reign of destruction. It is a dark and ominous dungeon-like place located deep beneath the ocean, a few jagged rocks can be seen rising from the sea where Tartarus lies. There is also a small island filled with jagged mountains where the Titan Prometheus is imprisoned and each day a giant eagle comes to eat his liver.
Elysian Fields: The final resting place for the good and the heroic. It is a paradise filled with beautiful valleys and temples. This part of the Underworld is protected by Zeus, but Hades attempted to win it from Zeus in a bet in the episode "Hercules and the Driving Test", but he lost the bet however.
Sora, Donald, and Goofy emerge in the Underworld when they first arrive, meeting Megara, who is on a quest to convince Hades to stop bothering Hercules. When Sora confronts Hades, he summons Auron, who quickly switches to Sora's side. Hades cannot be hurt in the Underworld, so the team attempts to escape, and Auron and Sora are forced to take on Cerberus before they can leave the Underworld. Back up top, Hercules offers the junior heroes the Olympus Stone, which stops the Underworld from draining all their strength, however, the stone has been stolen by someone in a black cloak. Looking for the stone, they encounter and have their first confrontation with Demyx. The team then rescues Meg with the help of Hercules and Pegasus before opening another gateway with the Olympus Stone.
When the team enters the world for a second time, the Underdrome has been unlocked, presumably when Sora used his Keyblade to open a wall during the first visit. Hades kidnaps Meg to bait Hercules into the games. The group enters the Pain and Panic Cup, and the final competitor is Auron, who is being controlled by Hades. Hercules faces Auron alone while Sora, Donald, and Goofy retrieve the statue Hades is using to control Auron. By returning the statue, Auron regains his free will and refuses to fight. Hades, as Hercules' punishment, drops Meg into the River Styx and Hercules jumps in after her. Hades believes he is dead, but Hercules returns for the final battle with Hades, a true hero at last. They defeat Hades, everyone is reunited, Auron is freed, and Sora, Donald, and Goofy are immortalized as constellations to mark them as true heroes, much to Phil's disbelief.
After Emma, Hook, Regina, and the group return to Storybrooke after their visit in Camelot, a Fury, a dangerous creature absorbing life-force, attacks Robin Hood and tries to take him to the Underworld. A mysterious ferryman Charon waiting the soul. However, he vanish when Regina, combining her force with her allies, eliminates the creature.
After Hook is killed, Emma, Rumplestiltskin, Regina, Snow, Charming, Robin Hood, and Henry enter the Underworld to bring Hook back from the dead. The Underworld resembles Storybrooke with the exception of the broken clock tower and the charred remains of Emma's car. Cora serves as the mayor until she fails her assigned mission from Hades. After her demotion, Cruella De Vil is chosen as the new mayor.
It was revealed that Hades made the Underworld look like Storybrooke for Zelena, so she could have her own Storybrooke to rule, as she felt Regina got everything.
Regina and the group have dubbed the Underworld as "Underbrooke".
According to the creators of Once, the Underworld is not Hell, but rather a place for people with unfinished business, which is why characters who are not villains such as Hook are there.