The Udefrykte (also spelled Uderfrykte or Udyrfrykte) is the name given to a legendary troll. Only two trolls have been recorded in history to go by the name. They are the spawn of the Uderfrykte Matron, also known as the Horror of Dive Rock.
An Udyrfrykte once lived in the ice caves beneath the mead hall of Thirsk on the island of Solstheim. He was described by the locals as "a remorseless monster with a taste for human flesh". According to legend, he was sealed inside his lair for centuries, until he escaped in 3E 427 during the Bloodmoon Prophecy, when a pillar of fire emanated from Lake Fjalding. The fire melted the ice covering the entrance to the ice caves on the shore of the lake; the Udyrfrykte then attacked the mead hall, slaying the warriors within and leaving few survivors. Svenja Snow-Song survived the attack, and urged the Nerevarine to slay the monster. The Nerevarine killed the Udyrfrykte and took his heart as a trophy. Svenja then appointed the Nerevarine as the new chieftain of Thirsk.
Svenja married Agnar the Unwavering, the next chieftain following the Nerevarine's departure. Haunted by memories of the Udyrfrykte's attack, she and Agnar set out to find the ancient Uderfrykte Matron, the mother of the Udyrfrykte. The creature was described as "a giant, troll-like beast that seems to waver and shimmer in the cold", "like the feral form of winter itself". Unlike her offspring, the Matron moved from location to location, preventing her killing or capture. It was completely immune to the cold, but was extremely weak to fire. The couple tracked the monster from the remote highlands of Skyrim into the Jerall Mountains. They confronted the Matron on the Cyrodiil side of the border, near Dive Rock, the highest natural observation point in the Imperial Province. The monster devoured Svenja and killed Agnar. The Champion of Cyrodiil stumbled upon their campsite in 3E 433, and managed to kill the monster.
Another Udefrykte lived in the enclosed ice caves beneath the settlement of Dawnstar in Skyrim. The monster broke through into the abandoned Dark Brotherhood sanctuary beneath the city. It was slain by the Listener in 4E 201, and the ice caves were then resealed when the Dark Brotherhood reclaimed the sanctuary.