Poseidon uses his mermaid daughter Ursula to lure pirate ships to rocky shores where they meet their demise. However, Ursula becomes sick and tired of her singing voice being wielded as a weapon by her father, and plans on leaving the ocean. To stop her, Poseidon strikes up a deal with Captain Hook, who uses a magic seashell to capture Ursula's singing voice so that she cannot pursue her dreams of singing to make people happy, and will have no reason to leave the seas. However, Ursula finds out that it was Poseidon's plans and uses his trident to transform her tail into tentacles. Inspired by the ancient deity Ursula, who she was named after, she decides to become feared.
When Ursula arrives in Storybrooke, allied with Maleficent, Cruella and Rumplestiltskin, Hook sends the mermaid Ariel to find Poseidon. Ariel succeeds in bringing Poseidon to Storybrooke, and Poseidon convinces Ursula to give up her conjoined plans with the Dark One and the other Queens of Darkness. Ursula's singing voice is returned to her, and Ursula abandons her plans, forgiving her father. Poseidon and Ursula decide to return back to the ocean.
- Ursula (daughter)
- Poseidon is based on the deity of the ocean from Greek Mythology,
- He also has some elements of King Triton from The Little Mermaid.
- He was not only known in the Enchanted Forest, but on Earth he was worshipped by the Ancient Greeks and Romans as a god.
- There is an area of the ocean near Arendelle known as "Poseidon's Boneyard", which is where Anna and Elsa's parents died at sea.
- S4, E15: "Poor Unfortunate Soul"