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The Dire wolves were a large pack of wolves that served under the Witch-king of Angmar. They assisted him in retaking Angmar from the Black Númenóreans. They were all killed in this battle, though more came to the Witch-king later. The wolves were possibly sentient. Dire Wolves are not considered canon, as they only appear in the video games. In Rise of the Witch-king, they can be upgraded with Banner Carrier and Spiked Collars.
An extinct animal of the same name existed in the real world - "dire" is from the Latin meaning "Fearsome".