Lord of a Thousand Suns

"Lord of a Thousand Suns" by Poul Anderson is a science fiction story that tells of an immortal being from an old civilization called Vwyrdda who ends up stuck inside a human soldier named Lieutenant John Laird during a war between Earth and the Janyards. The story explores what happens when two very different beings share one body, as Daryesh, the immortal, copes with being mortal while also dealing with Laird's memories. Laird is on a quest to find strong weapons on Vwyrdda when he accidentally wakes up Daryesh, who knows secrets that both sides of the war want. The two personalities have to figure out how to work together to survive attacks from the Janyards, which could change everything about the war and raises important questions about what it means to exist, have power, and about what the future holds.