Death

You may be looking for the Eternal or The Mefistolean Avatar.

Death was the cessation of life. The Ninth Doctor told Rose that "everything dies". (TV: The Parting of the Ways) Every single death caused a tiny fracture in reality. (TV: The Woman Who Lived) In 2011, three hundred thousand people died on Earth every day. (TV: The New World)

The Eternal, Death, was a personification of the concept. (PROSE: Timewyrm: Revelation) Kali was the Hindu goddess of death. (COMIC: The Swords of Kali) On Earth, and later human colonies, the skull was a generalised symbol of death, and the idea took this form as an emoji: 💀. (TV: Smile)

When discussing the idea of a "planet ruled by zombies", the Fourth Doctor remembered the quote that "the living are just the dead on vacation." (TV: Destiny of the Daleks) Voicing her observations on the human lifespan, which she found "hilarious[ly]" short, Missy stated that human beings were born dying, remarking "You know from the minute you slop out, you're rotting, decaying." (TV: Death in Heaven)

As Arboretans existed partly outside of time, when one died, their consciousness travelled back to their birth and they lived their life over again, trying to fix mistakes they had made until they achieved the best life possible. They called this "the path of perfection". (PROSE: Festival of Death)

In 2011, for several months following the Miracle Day, every human on Earth simply stopped dying. It was caused when the Three Families fed the immortal Jack Harkness' blood into the Blessing, which in turn relayed the template to everyone on the planet in seconds. Death was restored when Jack's mortal blood was fed into the Blessing and the original template was restored. All across the world, those who were unable to die yet should have died, including Geraint Cooper and a little girl, all took one final collective breath before dying. For them, the return of mortality was a mercy for them. In the case of others, like the Mother and the Cousin who were responsible for the whole thing, it came back with a vengeance. (TV: The New World, The Blood Line)

In the final stages of mental shutdown, when physical shutdown would commence, brainsteam death was a sign closely followed by complete respiratory collapse. Soon, liver failure and renal collapse would signal general systemic failure, leading to death within an hour. (PROSE: Anachrophobia)

Death and the Doctor

Clive Finch warned Rose Tyler that the Doctor had one constant companion: death. (TV: Rose) Indeed, several of the Doctor's adventures resulted in the deaths of all who participated except for him and his companions. Among these were the Fourth Doctor and Leela's conflict with a Rutan scout at Fang Rock, (TV: Horror of Fang Rock) the Fifth Doctor, Peri and Erimem's conflict with Jarra To at the Axis, (AUDIO: The Axis of Insanity) their later visit to YT45, (AUDIO: The Mind's Eye) the Sixth Doctor and Frobisher's visit to Eugene's World, (AUDIO: The Holy Terror) the Seventh Doctor, Ace and Hex's conflict against the Daleks on Bliss, (AUDIO: Enemy of the Daleks) the Ninth Doctor, Rose Tyler and Jack Harkness in the Battle of the Game Station, (TV: The Parting of the Ways) the Eleventh Doctor, Amy Pond and River Song's conflict against the Weeping Angels on Alfava Metraxis (TV: Flesh and Stone) and the Eleventh Doctor, Amy Pond and Rory Williams' visit to House's bubble universe. (TV: The Doctor's Wife) In one unusual case, the participants all died after the Doctor departed, (AUDIO: Brotherhood of the Daleks) and in another, they all died before he arrived. (COMIC: Death to the Doctor!) On one planet, the Fourth Doctor suspected that his friends the Guardian Angels had all died, but left without confirming this. (COMIC: End of the Line)

In other cases, the Doctor's adventures resulted in the deaths of all but a few of the participants. These included the Fourth Doctor and Sarah Jane Smith's conflict against Sutekh in 1911, which only Ahmed survived, though another account said he did not, (TV: Pyramids of Mars) the Fifth Doctor's visit to Sharnax, which only Kylo survived, (AUDIO: The Burning Prince) the Fifth Doctor, Tegan and Turlough's conflict against the Silurians and Sea Devils in 2084, which only Bulic survived, (TV: Warriors of the Deep) the Fifth Doctor and Peri's visit to Androzani Minor, which only Timmin survived, (TV: The Caves of Androzani) the Sixth Doctor and Peri's conflict against the Sontarans in Spain, which only Anita survived, (TV: The Two Doctors) the Sixth Doctor and Peri's visit to Nekros, which only Takis and Lilt survived, (TV: Revelation of the Daleks) the Seventh Doctor and Mel's visit to Puxatornee, which only the Slithergees survived, (AUDIO: Flip-Flop) the Seventh Doctor and Ace's conflict against the Gods of Ragnarok on Segonax, which only Kingpin, Mags and a stallslady survived, (TV: The Greatest Show in the Galaxy) the Seventh Doctor, Ace and Hex's conflict against the Cybermen in 2021, which only Mark Mathias survived, (AUDIO: The Harvest) the Eighth Doctor, Charley and C'rizz's visit to the Cube, which only Gorden Latch and the brain-worm survived, (AUDIO: Something Inside) the Tenth Doctor and Donna Noble's conflict against the Pyroviles in Pompeii, which only Caecilius, Metella, Evelina and Quintus survived (TV: The Fires of Pompeii) and the Tenth Doctor's conflict against the Flood on Mars, which only Mia Bennett and Yuri Kerenski survived. (TV: The Waters of Mars)