Extra life

The extra life is a power-up which is introduced in Doom Eternal.

Tactical analysis
If the player has an extra life, it will automatically be consumed when their health reaches zero, fully restoring health and offering a brief period of invincibility rather than forcing the player to restart from the last checkpoint; extra lives thus work similarly to the Saving Throw rune from Doom (2016). If the player is using the Savage Throw rune, extra lives will be consumed before the rune is used.

Extra lives are marked on the automap as Doom Slayer helmet icons; in game, they take the form of a helmet with "1UP" displayed above it. Unlike most major collectibles, they respawn when replaying a mission.

Doom Eternal includes an Extra Life Mode that can be applied to any skill level except Ultra-Nightmare; in this mode, running out of lives will end the campaign, forcing the player to restart from the beginning. Extra lives do not appear in Ultra-Nightmare at all; they are either removed entirely or replaced with spirit armor.

A cheat code that grants unlimited extra lives can be found in the Hell on Earth level. It is still possible to earn extra lives for the current save file when using this cheat even though the counter is not visible (replaced with an infinity symbol), making it a good way to accumulate lives.

The Ancient Gods
Part One introduces a rare 2UP collectible, which adds two extra lives instead of one. It also includes a support rune called Take Back - when active, if an extra life is consumed then the player can get that life back by quickly killing the monster that dealt the fatal blow. The offending monster will be marked with an extra life icon; stronger monsters will allow more time to kill them.

Horde Mode
In Horde Mode, players begin with three extra lives and can collect more as they progress; running out of lives will force the player to start again from the beginning. On completing the game, every remaining extra life is converted to 1,000 bonus points each.

Horde Mode introduces a 3UP item that is given for successfully completing a bonus round, and awards three extra lives. 1UP items are given at the end of each arena round once all the weapons are obtained; they are also given if a bonus round is not completed successfully.

Achievements
The following achievements involve collecting extra lives, and will also tick off their corresponding milestones:
 * Extra Extra Lives - Pick up 20 extra lives in the campaign in a single save slot.
 * Running Up the High Score - complete the campaign in Extra Life Mode with 10 extra lives in your inventory.
 * 1-Upsmanship - Complete The Ancient Gods, Part One in Extra Life Mode with five extra lives in your inventory.
 * Live Die Reload - Complete The Ancient Gods, Part Two in Extra Life Mode with five extra lives in your inventory.
 * Max Credits - Complete Horde Mode with 20 extra lives in your inventory.

Trivia

 * A similar concept was introduced in Doom (2016)'s Arcade Mode, where extra lives take the form of special Doomguy collectibles. Rather than allowing play to continue immediately without a checkpoint, they instead allow the game to be continued from the previous checkpoint. This is in contrast to the fact the current run normally ends immediately upon death.
 * Extra lives were originally intended to be part of the classic Doom and were included in Tom Hall's Doom Bible design for the game as a hold-over from Wolfenstein 3D. They were partially implemented even until the press release beta, where a life count was displayed as part of the status bar, and were awarded by collecting soulspheres. However, this concept, along with scoring and treasure items, was removed before release.