He saves the world, but kills himself by releasing Juno. According to lead designer Steve Masters, Desmond's story ends in Assassins Creed III: "What we're trying to do is bring some finality to Desmond's story.

What happen to Desmond Miles?

After the three exited, Desmond walked up to the orb. Juno watched as he touched it with his hand, his body shook violently for several moments and shone a bright gold; though, after a brief time, Desmond fell to the ground, sacrificing his life to protect the planet from the Second Disaster.

Is Desmond Miles alive Valhalla?

In Assassin's Creed III, Desmond is able to find the Isu device created to stop such an event from happening again, though its use would require Desmond to sacrifice his life. ... Despite Desmond's death seven entries prior to Assassin's Creed Valhalla, the echo of his sacrifice is still felt in the game's narrative.