Sakura's quick and dirty elaboration on this, as it's not in the chronology yet:
Usagi is dying by the point of old age but they feel she is the only one who can save the galaxy and so some of the senshi offer their life force to her to revitalize her because they believe only she has the key to victory. This is an echo to the legions who gave their lifeforce to galaxia in our version of the sailor wars. (Not every senshi will offer, some will feel her time must pass and some just wnat to live a bit longer. (they are all slowly dying, so it's not as big a boost as one might think) Usagi, believing that she IS the only one who can do anything, but distraught over her frailty, will take the offering, and become sailor cosmos. But can she live with it. And how will the others live with her? This is why cosmos was so messed up in the head in the manga and wanting the war to end.

• The Little Things
  Mother and Daughter console each other, realizing all the things never said to their late friend Furu.
  By Doctor Xadium


• Heartless to Heartless
  Mango Kattan is immortal, ageless, and grieving the loss of her lover, Furu.. When Yaijinden offers her a way to end it all, will she accept?
  By Yaijinden


• The Tears of the Flame
  Xadium must atone for his crime born out of love; but will the penalty cost him his marriage-- and his life? At the same time, Hino Rei and rei.bot face her destinies.
  By Nitemare_Angel and Doctor Xadium


• Decisions
  As Serenity's final breath nears, she is forced to a final, grim decision; she can either let her life finally end, or she can sacrifice others to protect the Galaxy. Will she make the right choice?
  By Nitemare_Angel
• Requiem for the Lost
  Two women bound by fate meet to discuss the horrible tie that binds them together-- the death of their beloved friend.
  By Doctor Xadium


• Remnants
  A search amongst the ruins finds interesting things
  By Tuxedo Rainbow


• The Final Legacy
  On a lone starship 11,000 years into the future, Xadium and Minako discover a little girl carrying a terrible secret.
  By Doctor Xadium


• Metamorphosis the First
  On the desert world Arrakis, the Khadi Yaijinden makes a startling choice...
  By Yaijinden


• Hatred of Angels
  Minako and Xadium arrive on the war-torn world of Caliban, and Minako makes a decision that will forever shape the destiny of her friends.
  By Solarchos


• Song of the Heartless
  Sakura journeys to the end of Earth, where one man sits alone in the Burning Sands in contemplation.
  By Doctor Xadium