The Nattmara

The Nattmara is the main antagonist of Frozen II: Forest of Shadows. It is a manifestation of Elsa’s fears which ran amuck Arendelle. It was eventually defeated when Anna helped Elsa overcome her fear.

Beginning
When Elsa was eight years old she had a nightmare that left her entire room covered in snow. To prevent her from having nightmares again, her mother taught her to imagine that she’s wrapping her fears up in a ball and give it to a constellation. As her favorite constellation was Ulf the wolf, Elsa would imagine that she was feeding her nightmares to it. While this stopped her from having nightmares afterwards, her expelled fears would mutate into a Nattmara in the form of a white wolf.

As Elsa represses her nightmares, the Nattmara would haunt Anna’s dreams to feed on her fears and grow stronger. Anna first had a nightmare where the giant wolf was chasing her sister and devouring her after her accident in the ballroom that led to Elsa and Anna’s separation.

Forest of Shadows
One week before the story, Anna became distraught that Elsa was planning to go on her grand tour without her. Fearful that it was because Elsa didn’t need or trust her, the Nattmara returned and continued to haunt her dreams and feed on her fears, as well as simultaneously continuing to devour Elsa’s fear of not being a good enough queen.

Growing stronger from the fear of two scared sisters, the Nattmara began to take form in the real world as a blight. Over the next few days animals began to fall asleep (accompanied by white fur) and wouldn’t wake up. Shortly after crops began to wilt and Arendelle's food rations began to deplete. This both caused the entire kingdom to be placed into a state of panic and caused Elsa to become even more afraid.

With this, the Nattmara was finally strong enough to take form in the physical world, appearing right behind Anna (coincidentally right after she read a “spell” designed to make dreams come true, making it believe it was her fault).

The Nattmara then proceeded to chase Elsa to feed on her fear. Everyone who came into contact with the wolf while sleeping would be placed into a deep sleep filled with nightmares and erupt with a frenzy when awoken.

The Nattmara would chase Elsa and the gang to the black mountains when they were trying to find the REVOLUTE blade, believing it could kill the creature. Whenever Elsa would try to fight it, the Nattmara would evade her attacks by turning into black sand and grow stronger.

After finding the Revolute blade in the tumulus, Elsa tried to fight the Nattmara, which was roaming Arendelle, using the blade. This attempt was unsuccessful and resulted in the blade being crushed by the Nattmara. After this, Nattmara took control of Elsa and imprisoned her in a nightmare.

Anna returned to save Elsa, where the Nattmara was circling Elsa as black sand, causing her nightmares of failing as Queen and feeding on her fear. Realizing the Nattmara's true nature, Anna told Elsa that she is a great queen and her role model.

As Elsa overcame her fear the Nattmara shrank to the size of a puppy before disintegrating, leaving a single speck of black sand behind, which Elsa then encased in an ice-cube.

Trivia

 * Olaf named the Nattmara Fredrick.
 * The Nattmara is a mythological creature of Scandinavian origin.
 * Despite Anna learning that the spell she thought was used to summon that Nattmara was a useless poem, Elsa was still afraid that the Nattmara would still remain permanently after three days.
 * The Nattmara's existence contradicts a few other Frozen stories: Elsa previously had nightmares for a whole week in the comic story “restless week” and she had already confided to Anna her fears of not being a good queen in “Phantoms of Arendelle”. Also, Elsa and Anna already went on a royal tour together in "Across the Sea."