Underworld

The Underworld is the realm of which the souls of the deceased with unfinished business live out the rest of eternity.

Before the Third Curse
When defeating King Arthur and Zelena, winning over DunBroch's loyalty and finally being crowned Queen, Merida is gifted a magical ale by the Witch, giving her the ability to communicate with her deceased father, whose soul spends the rest of eternity down in the Underworld. Fergus is proud of what his daughter has accomplished, and the two share a heartfelt goodbye.

Season 5
Due to not paying the price of magic back in Camelot, Regina risks losing Robin Hood forever when a fury arrives in Storybrooke to claim his life. She works to stop the demon, and with the help of Mary Margaret, David, and Leroy, she is able to stop it from sucking out Robin's life energy, sending it and the Charon who waits in the town pond nearby, on their way.

Hook, who has come to realize Emma made him a Dark One back in Camelot, gives into his dark urges and starts plotting to snuff out the light. He goes to the Storybrooke pond, and there, summons a portal to the Underworld, thus allowing the Charon to bring all the other deceased Dark Ones to town so they can help him with his plan.