Singing Spell

Singing Spell, also known as Singing Wish, is a spell that allow the caster to magically enhance, remove or restore their singing voices.

It featured on ABC's Once Upon a Time. It first appears in the "Poor Unfortunate Soul" of the Season 4.

Description
Singing spells can be cast by various different means, including a deity's enchantment, a potion or a fairy granted wish. However, a Dark One like Rumplestiltskin is immune to being forced to sing by this spell.

Before First Curse
Born to King Poseidon and his unnamed wife, Ursula is named after a sea goddess of the same name. As a child, she is taught to sing from her mother, who used to sing at Glowerhaven. After her mother is killed by a pirate, Poseidon holds a grudge against humans, and he makes his daughter lure sailors at sea to their doom with her singing voice. Ursula complies, nearly killing the crew aboard the Jolly Roger, but she lets them go at the last moment. While Poseidon is disappointed in her actions, she argues that her singing is the only memento of her mother, and doesn't wish to use her gift to harm others. At her insistence that not all humans are bad, he orders her to obey his wishes as long as she lives in his kingdom. She rebels, stealing a magic bracelet from his vault, which gives her human legs. With plans of going to Glowerhaven, she begins singing at a pub to earn gold for the trip.

After Ursula captivates an audience with her voice, Hook, captain of the Jolly Roger, befriends her and offers to take her to Glowerhaven. However, she later learns from Hook that her father asked him to trap her singing voice within an enchanted shell in exchange for squid ink, which he desires for vengeance against an evil man. Instead, Ursula steals the ink from Poseidon's vault for him just as her father arrives to take the procured item away. Angered that his only hope of revenge has been crushed, Hook removes Ursula's singing voice so Poseidon can never use his daughter to sink another ship again. Shattered at this betrayal, Ursula decides she no longer wants to be a mermaid and uses her father's trident to transform her tail into tentacles. She claims that the whole sea will be at her mercy because of her newfound power and that even her father will come to fear her. ("Poor Unfortunate Soul")

When Snow White wishes for her unborn daughter to have the chance of a happy ending, the Blue Fairy casts a spell to make everyone sing the song they have in their hearts. This results in Snow and Charming, Hook, and Regina singing, along with everyone else in the Enchanted Forest, in addition to the spell also affecting Zelena in Oz. Their songs include: Powerful Magic, Revenge Is Gonna Be Mine, The Queen Sings, Love Doesn't Stand a Chance and Wicked Always Wins. In an attempt to make Rumplestiltskin see that she is more powerful than her sister, Zelena creates a spell to seal away the singing enchantment, discreetly giving it Regina in a box, in the hopes of later revealing to Rumplestiltskin that the spell was her creation. The songs are captured but later put in Emma's heart by the Blue Fairy, who erases the memories of everyone to ensure the songs remained safe because Emma will need them for the future when she faces a prophesied battle on her own. ("The Song in Your Heart")

After Second Curse
Desiring restore Ursula singing voice, which he once sealed into an enchanted shell, Hook summons Ursula's father Poseidon, who can restore his daughter's voice. Poseidon apologizes to Ursula for using her singing voice for bad deeds and he expresses regret over not cherishing her gift to honor her mother's memory. After regaining her voice, Ursula fully reconciles with her father and decides to go home with him. Before leaving, she tells Hook about Mr. Gold's scheme to dethrone Emma as Savior and make her evil. Ursula also explains that Emma gave everyone their happy endings in this world, and once she is not the Savior anymore, only then can the Author give the villains what they want. ("Poor Unfortunate Soul")

Years later, Emma is about to have her heart crushed by the Black Fairy, but the evil woman finds herself unable to because of the song hidden within it. With this song to strengthen her, Emma harnesses enough of her own magic to break the vile woman's freezing spell on her family. ("The Song in Your Heart")

Trivia

 * Singing spells (at least their appearance) are based on a similar spell from Disney's "The Little Mermaid".