Thralls are enemies encountered in Tomb Raider: Underworld, and are found in the Croft Manor, Southern Mexico, Jan Mayen Island and Arctic Circle levels. They also feature in the DLC levels, Beneath the Ashes and Lara's Shadow.
Thralls are deceased individuals whose corpses have been reanimated by the substance eitr. They serve primarily as the guardians to Thor's Gauntlets, Thor's Belt and Mjölnir, as well as the guardians of Helheim. Thralls are described as being invulnerable to all weapons, except Thor's Hammer, as long as their skeletons are intact. As such, after a Thrall is knocked out, Lara should stomp on the Thrall to shatter its skeleton to prevent it rising again.
There are four different types of thralls in the game:
- Tiger/Panther Thralls - the skeletons of deceased big cats reanimated by eitr. Tiger Thralls are first encountered beneath Croft Manor, but they originally guarded Thor's Gauntlet in Thailand. Panther Thralls appear in Southern Mexico, guarding Thor's Belt.
- Mayan Thralls - the skeletons of Mayans who guard Thor's Belt. They spit eitr at intruders, and also make attempts to throw them into the eitr to join their ranks as guardians of Thor's Belt.
- Viking Thralls - first encountered on Jan Mayen Island, Viking Thralls are largely the same as their Mayan counterparts, but are tougher to kill.
- Yeti Thralls - the corpse of a Yeti is difficult to identify once it has lost its hair, but these Thralls were clearly Yeti when they were alive. Much larger and tougher than the other thralls, the Yeti Thralls are also much slower, and tend to appear among groups of Viking Thralls. Yeti Thralls can also withstand a direct blow from Mjolnir. They are found guarding Helheim, although a solitary Yeti Thrall can be found on Jan Mayen Island.
- Knight Thralls - Knight Thralls resemble the Templar Knights, and are roughly the same as Viking Thralls. They are found beneath Croft Manor, guarding the Eitr Stone.
- Super Yeti Thralls - These Thralls are similar to Yeti Thralls, but possess heavier armour and are stronger.