The primary role served by the demon keys is to unlock the full potential of the Unmaker, an ultimate weapon of demonic origin. Each artifact acquired will increase the power of the weapon to its next level. The three possible increased power levels are:
- Doubled firing rate
- Doubled laser output
- Tripled laser output
None of the three artifacts grants a specific power; rather, the power of the Unmaker is based only on the number of artifacts possessed.
A secondary function of the demon keys is their utility in the final level of the game, MAP28: The Absolution. Each key has a corresponding switch near one of the three monster teleport destinations on the map. If the corresponding key is possessed, that switch can be activated to seal off that portal and instantly kill all the monsters that have not already made it through (any still within the confines of the portal's exit as it closes will also be crushed).
If all three artifacts are possessed, this allows skipping most of the major battle which precedes the fight with the Mother Demon. If a corresponding artifact is not possessed, attempting to activate the switch gives the somewhat mysterious message, "You lack the ability to activate it."
In the Lost Levels expansion of the 2020 re-release, these artifacts are required to progress through MAP39: Final Judgement in order to summon the Sister Resurrector.
Demon keys are lost when restarting a level after death.
Each of the three artifacts is found on one of the game's three normal secret levels:
In the TC version, one of the three artifacts was moved to MAP34: Crisis.
The demon keys are shaped like pentagrams with a beastly face. The first artifact is a yellow/orange hue, the second is a deep purple, and the final is cyan. Interestingly, the areas in which the keys are found share their color scheme, as can be seen in the gallery. Collecting each of the keys gives the player a different message. Getting the message can be difficult, however, because the keys also rest within secret areas, and the message "You found a secret area!" often immediately overwrites them.
- Orange Key: "You have a feeling that it wasn't to be touched..."
- Purple Key: "Whatever it is, it doesn't belong in this world..."
- Cyan Key: "It must do something..."