The Darkness

The Darkness, and its host the Dark One, is a powerful entity in Once Upon a Time in Middle-Earth.

Background
The Darkness is created inside of Nimue, who drank from the Holy Grail, making the Darkness immortal. Nimue crushed the heart of the man who murdered her family, friends and their entire villages, thus becoming the first Dark One.

Season 4
In "Operation Mongoose Part 2", the Darkness becomes unattached to Rumplestiltskin and escapes the imprisonment of the Sorcerer's Hat. The Darkness begins to tether itself to Regina and begins tethering itself to Bilbo Baggins. However, Emma takes the Dagger and lets the Darkness be stored inside of her to spare Regina and Bilbo. In contrast, the Darkness also attaches itself to Bilbo, and as they absorb the Darkness, Bilbo and Emma become Dark Ones.

Season 5
Both Bilbo and Emma work tirelessly to rid themselves of the Darkness, as both of them split the burden in half and both are now immortal. Later, Emma makes Killian a Dark One, without taking Bilbo's or her family's advice.

Known Dark Ones

 * Nimue †  (Original Dark One)
 * At least eighteen unnamed Dark Ones †
 * Gorgon †
 * Zoso †
 * Rumplestiltskin / Mr. Gold
 * Emma Swan  (Darkness removed)
 * Bilbo Baggins (Darkness removed)
 * Killian Jones  (Darkness removed)