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:
 * A flat circle about two hundred feet across,
 * A roughly square multi-level arena with walls and 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.

The square arena is more common than the other two. All Trials limit the player to only one weapon, which may or may not have a mod fitted (if so, it will be in its default state with no upgrades), and all have time limits of one kind or another: either a static limit, or a short timer which is increased by performing certain actions. Equipment is not allowed. Other common rules that may apply include having 1 health, or in a couple of cases, having a limited amount of time to move after a kill before losing vertical movement (jumping is still possible in this state). Some trials are more difficult than others, and hence may require special strategies. The jump boots still work in the Trials, which is important for finishing the obstacle course trials, and they help with the Seek and Destroy trial, which requires the player to kill three Hell Knights using the Death from Above Glory Kill specifically.

Savagery is generally regarded as the worst choice of Rune since its only effect is decreasing the amount of time the Slayer is invincible for while performing a Glory Kill; it can, however, be useful for speed-running, and can be helpful if performing a Glory Kill runs the risk of the Slayer being surrounded during the animation. Dazed and Confused and Intimacy is Best are primarily useful for unlocks that require specific Glory Kills, with the latter good for enemies such as Barons of Hell that are hard to place in a stagger state without accidentally killing them. Of note is that Blood Fueled and Armored Offensive trigger on kills with the Chainsaw as well as normal Glory Kills.

Equipment Power's description does not really detail what the Rune does: while it is simply a damage buff for regular grenades, it upgrades the Hologram so its decoy Plasma Rifle deals actual damage, increases the hologram's duration and prevents it being destroyed by enemies. Siphon Grenades are no longer limited to refilling health: once the health bar is filled, they will switch to giving armor.

Kills in Rune Trials do not count towards achieving a weapon's Mastery.

Doom Eternal
Runes return in Doom Eternal, but are altered somewhat. While Runes are still found in the environment (this time as floating Makyr devices), there are no Rune Trials: on encountering one of the devices, the player can simply choose which of the nine available Runes that they wish to unlock. The first three Rune unlocks each unlock a Rune Slot in the inventory.

There is no longer a levelling system for Runes: they always operate in the same way. Also, all Runes are immediately available for selection, whereas in Doom (2016) some like Saving Throw were only available later in the game.

The Ancient Gods - Part 1 adds a new type of Rune called a Support Rune. These are only useable in the DLC. There are three available, which are equipped to their own slot which unlocks when the player acquires one of them. They cannot be equipped in the normal Rune slots.

Take Back cannot be equipped in Ultra-Nightmare or Extra Life mode.

Trivia

 * Saving Throw comes from the Dungeons and Dragons role-playing 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.

Runa (Doom4)