The three Demon Keys (sometimes called Demon Artifacts) are special items found only in Doom 64, used to upgrade the Unmaker. These upgrades come in the form of a faster and improved fire rate and an increase in damage dependant on how many keys the player has picked up.
The keys are golden, pink and blue in hue, and have the appearance of a glowering, horned demon head with an "X" etched into its center horn atop a stone pentagram similar to the graphic used in both the game's packaging and the background of the game's first option selection screen and end-of-level tally screen. One of each is found in the game's three main secret levels, MAP29: Outpost Omega, MAP30: The Lair, and MAP31: In The Void. They are also used in the final level Map28: The Absolution to disable the demon teleporters in the level.
While the keys themselves are not necessary for beating the game, both their ability to power up the Unmaker weapon and closing the teleporters makes the final level much easier to beat. The Unmaker's powered-up status is not lost when the keys are used.
The pickup messages are different depending on how many the player picks up, and in what order:
First one collected: "You have a feeling it wasn't to be touched."
Second: "Whatever it is, it doesn't belong in this world."
Third: "It must do something."