Acheron, also known as the River of Lost Souls, is the green river that runs through and around the Underworld. It condemns anyone who touches it to a fate worse than death, forcing them to endure eternal torment as who they are becomes a thing of the past.
After the Third Curse
Milah is thrown into the river. ("Devil's Due")
The River of Lost Souls is one of many rivers flowing from the center of Hades' lair. ("Souls of the Departed") The river remains there while Hook is tortured by the notorious God of Death. ("Labor of Love") When Hook refuses to chisel three of his friends' names onto gravestones, Hades threatens to throw him into the River of Lost Souls where he'll suffer eternal torment and lose himself in the process; a fate worse than death. He ends up suspending Hook on a chain and dangling him over the river, though eventually Emma Swan is able to save him after Gold secures her a boat to cross it. However, in a deal with Hades, Gold destroys the boat and throws his dead wife Milah into the water, lying and saying that Hades did it. ("Devil's Due") The river is seen in Hades' lair as he grabs some pages detailing his past from a separate stream, connected to a well on the surface. ("The Brothers Jones")
Belle does something she'll regret. ("Her Handsome Hero")
Hades gives some arrows to Gaston that are connected to the River of Lost Souls, wanting him to use them to shoot Gold. He tells Belle that if either Rumple or Gaston ends up pushing the other into the river - which is connected also to the town harbor - then he'll let her have ownership of her baby back. Ultimately, however, it is Belle who throws Gaston into the river when he tries to attack Rumple. ("Her Handsome Hero") Not liking how the heroes keep helping people in the Underworld move on, Hades spikes the soup in Auntie's Chicken & Waffles with water from the river. When Emily Brown drinks some, she is reduced to a puddle which Hades then collects in a jar which he later pours into the river. ("Ruby Slippers") James and Cruella threaten to push Emma and Robin into the harbor, only for David and Hook to arrive and rescue them. David and James get into a physical brawl which ends with David being forced to throw his twin brother into the River of Lost Souls. ("Sisters")
The souls attack. ("Last Rites")
Not wanting his father, Peter Pan, to rejoin him in the land of the living, Gold glamors a wine skin as a heart and fills it with water from the river. This means that Pan dissolves into nothingness when the "heart" is put in his chest, defeated once and for all. ("Firebird") Hook and Arthur sail across the river to get to the Underworld's version of the storybook - wanting to send a message to Emma using the book so that she can use the pages about Hades' past to find a way to finally kill him and end his reign of terror, which he is currently trying to ring across Storybrooke - only to be attacked by the souls within it. The first one grabs Arthur's ankle and tries pulling him in. Hook repels the one attacking Arthur with fire, and Arthur uses the same tactic to vanquish the one going after the book. Hook only just manages to catch it before it too goes flying into the river. The pirate surmises that with Hades having fled the Underworld, the rules must be changing, hence the ability of some souls to exit the river as ghostly, skeletal creatures. Soon though, Arthur decides that he shall take it upon himself to fix the broken kingdom of the Underworld, signifying that perhaps he will find a way to free the innocent souls from the river. ("Last Rites")