The Dark Energy Keys resemble black rings with four spikes on the outward edges. They float in the air and are surrounded by a swirling violet and blue aura.
The Dark Energy Keys only appear in the game's nighttime acts. In gameplay, after a Dark Energy Force Field has been set up, the player has to find three Dark Energy Keys that are placed in different locations throughout the current act, in order to proceed. These keys are usually found in locations leading up to the Dark Energy Force Field, such as behind doors or on different platforms, thus eliminating the need for the thorough player to backtrack in the level. When Sonic the Werehog makes physical contact with a Dark Energy Key, the player will automatically obtain it.
Once all three Dark Energy Keys are found and brought to the Dark Energy Force Field, the force field will be lowered, allowing the player to proceed with the act.
- Sonic Unleashed: Prima Official Game Guide for the Wii and Playstation 2, page 26.