The Dark Gauge is a gray-framed gauge that appears in the upper left corner on the HUD. Inside it is a red energy bar and on the right end is a hook.
In gameplay, the Dark Gauge is filled by committing dark deeds. These include:
- Damaging GUN agents and machinery
- Damaging architecture or nature
- Breathing Dark Mist
- Reviving Black Arms or Dr. Eggman's machinery
- Touching Dark Energy Core items
Once the Dark Gauge is filled, the player transforms into Dark Shadow, which grants infinite ammo for weapons, invulnerability and usage of the Chaos Blast. However, the Dark Gauge will steadily decrease while Dark Shadow is active and using Chaos Blast takes large chunks off it. Once the Dark Gauge runs out, Shadow reverts back to normal.
Due to the nature of certain missions of some stages, destroying Eggman's factions will occasionally provide energy for the Dark Gauge.