Jack o' lantern
From Halopedia, the Halo wiki
The Jack O' Lantern is an Easter Egg in Halo 2. It is simple to view, as you simply need to destroy a Scorpion, and stand on either the upturned turret or the main body of the Scorpion. Look at the gears on the connecting section, and there is a Jack O' Lantern drawn on blood. It may help to turn on your flashlight.
It is not known what the significance of the Jack O' Lantern is to the creator.
