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 Ledgendary icon for Campaign.
 * Halo 2 Skulls - In Halo 2, skulls are collectible within the Campaign. Once collected, a skull makes gameplay either more humorous, or more challenging. This 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, you 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, you 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: 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 fewer funny lines.
 * Halo: Combat Evolved Anniversary Skulls - At least one skull is known to exist in Anniversary.