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It's been confirmed that skulls will be included in the multiplayer mode similar to Horde in Gears of War, called Firefight. Possibly all of the skulls in Halo 3 will be included, and there will be a choice of some skulls (including IWHBYD) to pick from the start. Skulls are turned on either every wave or every few waves, and point multipliers are probably increased with the skulls turned on.