User can give their lives to shield others from harm, be it a life-threatening disease or injury or an apocalyptic force or event. This can be done either by using a specialized ability or ritual that will inevitably kill the user or by overusing one's natural capabilities until the strain on their bodies becomes fatal.
Death can occur immediately after the power is activated or at some later point, and the protection offered can either be defensive in nature (i.e. a protective charm/barrier that prevents harm or teleporting those the user wants to protect to another location) or offensive in nature (i.e. unleashing immense power/energy to seal or obliterate a dangerous entity). The duration of the protection can either be momentary, ending shortly after the user has died, or can last for much longer periods of time, possibly even for all eternity.
Depending on the power level of the user, along with the nature of the protection in relation to the threat it is being used against, the number of people that can be protected can be anywhere from an individual person/small group of people to a nation or continent to a whole planet or even an entire galaxy or universe. It may even be possible for immortal beings to use this power if they can sacrifice their immortality (which will eventually make them die from natural causes).
Commander Shepard (Mass Effect Franchise); via the Crucible
Previous generation Shuffle Alliance (Mobile Fighter G Gundam)
Sindel (Mortal Kombat franchise)
Zi Yuan (The Mummy: Tomb of the Dragon Emperor)
Kushina Uzumaki (Naruto)
Hiruzen Sarutobi (Naruto)
Minato Namikaze (Naruto)
Portgas D. Ace (One Piece)
Male Protagonist/Female Protagonist (Persona 3 Portable); via Great Seal
Juliet (Romeo x Juliet)
Queen Serenity (Sailor Moon); via the Silver Crystal
Black Magician (Yu-Gi-Oh!); via the effect of the Ectoplasmer Spell Card
Stardust Dragon (Yu-Gi-Oh! 5Ds)
Asuna (Sword Art Online)
Various characters (Sonic the Hedgehog)
The Crucible (Mass Effect 3)
The Forlorn Hope (Yu Yu Hakusho)
Aslan (The Chronicles of Narnia) willingly sacrificed himself to Jadis in place of the traitorous Edmund Pevensie to protect him and Narnia from The Deep Magic's otherwise unbreakable law and sanctions.
Axel (Kingdom Hearts) put his entire being into an attack to destroy a mass of Nobodies, sacrificing himself to protect Sora.
When Pandora used the effect of Ectoplasmer to sacrifice his Black Magician to damage Yugi's Life Points, Yugi's own Black Magician (Yu-Gi-Oh!) was also affected, enabling him to sacrifice himself to negate the damage.
BlackWarGreymon (Digimon Adventure 02) sacrificed himself to seal one of the gates that led to the Digital World, preventing Myotismon from using it to enter the real world.
Buffy Summers (Buffy the Vampire Slayer) throws herself into a dimensional portal to close it and save her friends and thus the entire world...
...sacrificing her life in the process.
Lucy (Elfen Lied) pushes her powers to the limit to heal Kouta's life-threatening wounds...
...causing her body to rapidly melt.
Once the Citadel was combined with the Crucible, Commander Shepard (Mass Effect franchise) could choose to give his/her life to destroy or dominate the Reapers, or even merge them with organic life.
After realizing that they were fighting their destined successors the previous generation of the Shuffle Alliance (Mobile Fighter G Gundam) sacrificed themselves to purify Domon Kasshu's corrupted friends, freeing them from the influence of the Devil Gundam and transferring their crests to them.
Desmond Miles (Assassin's Creed) sacrifices himself to activate the Eye...
...and create a shield to protect the Earth from an apocalyptic coronal mass ejection.
The Forlorn Hope (Yu Yu Hakusho) can grant any wish its user desires, but usually drains their life force completely in exchange.
Groot (Marvel Cinematic Universe) forms a shield of branches out of his own body to protect the other Guardians of the Galaxy from the Dark Aster's crash...
...which utterly destroyed his body.
Lady Yunalesca (Final Fantasy X) was the first summoner to use the Final Summoning to kill Sin, sacrificing her life in the process.
Lily Potter (Harry Potter) sacrifices her life to protect her son Harry from Voldemort, bestowing a powerful form of mystical protection upon Harry...
...rendering Voldemort incapable of even touching him without suffering excruciating pain.
The Lumas (Super Mario Galaxy) sacrifice themselves to close a massive black hole and save the universe.
Male and Female Protagonists (Persona 3 Portable); used their full potential to seal away Nyx, preventing the fall and saving the world from the embodiment of Death itself; doing this also cost them their lives after fulfilling their promise to their friends.
Sindel (Mortal Kombat) sacrificed herself for the sake of Earthrealm, creating a magical ward that prevented Shao Kahn from setting foot in it and thus conquering it.
Stardust Dragon's (Yu-Gi-Oh! 5Ds) Victim's Sanctuary power allows him to sacrifice himself to negate any effect that destroys cards, then revive himself at the end of the turn.
Vegeta (Dragon Ball) expended all of his life energy...
... to unleash a massive explosion that he hoped would destroy Majin Buu.
Through force of will and love for her unborn child, Portgas D. Rouge (One Piece) remained pregnant for over 20 months all the while not showing any signs of being pregnant until going into hiding; this resulted in her death after finally giving birth, essentially giving her life so her son could live.
Shinoda Chiaki (Darker than Black) sacrifices herself to save Hei from Jean's attack.
Tony Stark/Iron Man (Marvel Cinematic Universe) used the Infinity Stones to save the universe from Thanos and his army, an effort that cost him his life.
Harry Potter (Harry Potter) sacrifices himself to Voldemort in order to protect those at Hogwarts.
Son Goku (Dragon Ball) has sacrificed his life to protect others twice, once to save his loved ones and the world from Raditz...
...and once to save the world from Cell's self-destruction.
Asuna (Sword Art Online) takes the hit Akihiko Kayaba intended for Kirito, sacrificing herself for her lover.