Runes

Doom (2016)
Runes are special items earned by completing Rune Trials. Each rune can be equipped and placed into one of the Praetor Suit's three Rune Slots, though only one is unlocked by default. Completing more trials unlocks the remaining slots at four and seven trials respectively.

Runes can grant combat bonuses, from increasing item pickup radius to keeping enemies stunned for longer. Any rune can be placed in any slot allowing for customization to suit the player's preferred style.

There are twelve total Runes that can be found and unlocked, with two Rune Trials per level starting with the Argent Facility (Level 4) and ending with Lazarus Labs (Level 9).

Like weapons and weapon mods, Runes can also be mastered to provide greater benefits.

The Trials take place in what looks like a dark, nighttime, almost dreamlike area within Hell. There are three different trial arenas: The maze is more common than the other two, although all are used at least once. All Trials limit the player to only one weapon, which may or may not have an alternate mode of firing or allow for Glory Kills, and all have time limits of one kind or another. Other common rules that may or may not apply include having 1 health, or in a couple of cases, having a limited amount of time to move after a kill before grinding to a halt.
 * A flat circle about two hundred feet across,
 * A maze with several stairways that looks like part of a ruined castle,
 * A multi-part obstacle course over a chasm where the player must race to an altar.

In general, the later runes are more powerful than the earlier ones, and so are more likely to be used in the final levels of the game. The runes are more or less effective under different conditions, and call for different strategies. For example, some runes such as Armored Offensive are more useful in battles with many different enemies, whereas Saving Throw is more useful in boss fights, such as against the Cyberdemon. On harder difficulties like Nightmare, Rich Get Richer tends to be less useful, since the player is less likely to have over 100 armor for very long, whereas Glory Kill runes (Seek and Destroy, Savagery, Blood Fueled, and Intimacy is Best) might be more useful, enabling the player to dispatch enemies more quickly and thereby buying time.

Doom Eternal
Runes return once more in Doom Eternal, though this time instead of Rune Trials, they are found by exploring the environment.

Trivia

 * Saving Throw comes from the Dungeons and Dragons roleplaying game, which often makes players roll dice to have their characters avoid a harmful magical effect or save themselves from a dungeon trap.
 * A few of the runes could possibly be references to famous songs. The Dazed and Confused rune could be named after the Led Zeppelin song of the same name. The same is true with the rune named Seek And Destroy, which probably is a reference to the Metallica song of the same name.
 * The description for the Intimacy is Best rune is easily misinterpreted, making it seem less useful than it is. In fact, it makes demons stagger more easily from less damage than they would otherwise require, which is only clear from the description of the Mastery.

Runa (Doom4)