Squid Ink

"It wasn't the quill itself, but the Ink, what captured the Dark One... Harvested from the rarest species of squid, from the bottom of the bottomless ocean. Imposible to find, unless you're a mermaid, or me."

- Mr. Gold to Henry Squid Ink is a magical substance featured on ABC's Once Upon a Time. It first appears in the fourth episode of the first season.

Before the Curse
At some point in the past, the Squid Ink is obtained either by a mermaid or by Rumplestiltskin. ("Into the Deep")

The Blue Fairy created a magical red quill that could freeze Rumplestiltskin if he wrote with it using the ink. With the quill and ink, Cinderella tricked Rumplestiltskin into signing a fake contract. Now paralyzed, he is transported to his cell. ("The Price of Gold")

Rumplestiltskin took a jar of the ink with him to his prison. In exchange for information about the Dark Curse, Snow White told him her unborn child's name, Emma. Because he created the curse and knew how it could be broken, he was aware of the power of Snow White and Prince Charming. In order to be sure he would remember her name, Rumplestiltskin wrote Emma's name down repeatedly using the Squid Ink. ("Pilot")("Queen of Hearts")

After the Curse
In Storybrooke, Mr. Gold tells Henry to tell Aurora while in the Netherworld that the best way to stop Cora is the same way he was stopped, with the ink. By means of Henry and Aurora he informs the group that there is a jar of the powerful ink back in his cell. ("Into the Deep")

When Mary Margaret, Emma, Aurora, and Mulan arrive at Rumplestiltskins cell, they find the jar to be empty, but the paper with Emma's name is there. Cora and Captain Hook trap them in the cell and they grow desperate. Suddenly, Mary Margaret remembers Cora practicing Magic from her Book of Spells; she blows the ink off of the parchment, it forms a cloud, and the enchanted metal bars disintegrate. The four women then arrive in time to prevent Cora and Hook from entering the portal. ("Queen of Hearts")

Trivia

 * It first appeared in "The Price of Gold", but it wasn't mentioned until "Into the Deep".