Skulls

Skulls can refer to a number of different topics in the Halo franchise and universe, including:

Skull - Human skulls are usually used in gametypes such as Oddball or Headhunter as well as being used as a collectible/game modifier in Halo 2, Halo 3, Halo Wars and Halo: Reach. The Sangheili skull is most iconic for being on the Legendary icon for Campaign.
Halo 2 skulls - In Halo 2, skulls are collectible within the Campaign on Legendary Difficulty. Once collected, a skull makes gameplay either more humorous, or more challenging. This effect only wears off after turning off the XBOX or quitting the game on Halo 2: Vista.
Halo 3 skulls - In Halo 3, skulls are collectible within the Campaign. To use a skull, a player must collect one then turn it on. After collection, the skull is permanently available for use. Once activated, a skull makes gameplay either more humorous, or more challenging.
Halo Wars skulls - In Halo Wars, skulls are collectible within the Campaign. To use a skull, a player must collect one then turn it on. After collection, the skull is permanently available for use. Once activated, a skull makes gameplay either more humorous, or more challenging.
Halo 3: ODST skulls - Skulls are not found in the Halo 3: ODST campaign. A few can be claimed using the second disc (Halo 3 Multiplayer) and finding them on multiplayer maps in Forge mode. They unlock achievements in Halo 3 upon collection, but can be used in ODST's Campaign and Firefight modes to claim ODST achievements. The rest of the skulls can be found in Halo 3.
Halo: Reach skulls - In Halo: Reach, all skulls are available from the beginning. They are the same as the Halo 3 Skulls. Once activated, a skull makes gameplay either more humorous or more challenging. However there do appear to be some slight differences, such as the Grunt Birthday Party skull doing no damage, and the IWHBYD skull having more funny lines.
Halo: Combat Evolved Anniversary skulls - Skulls are once again a collectible within the campaign. Once collected, the skull is permanently available for use. There are no skulls that affect A.I behavior in the game in an effort to keep the game as close to the original as possible. The skulls are unable to be found or seen in Classic mode or split-screen Co-op.
Halo 4 skulls - Skulls are available in Halo 4 without claiming them in campaign levels.
Halo: Spartan Assault skulls - Skulls are available by default in Halo: Spartan Assault.
Halo: The Master Chief Collection skulls - All skulls are unlocked automatically in Halo: The Master Chief Collection.
Halo: Spartan Strike skulls - Skulls are available by default in Halo: Spartan Strike.
Halo 5: Guardians skulls - In Halo 5: Guardians, skulls are collectible within the Campaign. To use a skull,a player must collect one then turn it on. After collection, the skull is permanently available for use. Once activated, a skull makes gameplay either more humorous, or more challenging.
Halo Wars 2 skulls - Skulls are collectible within the Halo Wars 2 campaign, they can be activated before starting a level and can make gameplay easier, more challenging or humorous.
Halo Infinite skulls - In Halo Infinite, skulls are collectible within the Campaign. To use a skull, a player must collect one then turn it on when loading their save file. After collection, the skull is permanently available for use. Once activated, a skull makes gameplay either more humorous, or more challenging.

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