Reincarnated as a baby who kept his memory into the world of sword and magic, former shut-in mid age scientist sets on a mission to live a meaningful life, only to find out that his former boss who caused the accident and killed them both has been reincarnated aswell and is set on repeating the planet threatening experiment again. (Epic fantasy book novel)

Trying to shorten it futher. “After his boss kills them both, a former shut-in scientist reincarnated with his memories in a sword and magic world, must stop his menace finding a way to repeat the deadly experiment that could destroy the world.”

Problem is that you still use the word "world "twice, and I can't change the second "world" into Earth, because I don't kow if wer're talking Earth.Sure, I could end with the word "planet", but destroying a world=society/societies is different from destroying the whole planet.

Perfect! Thanx Richiev

"After his boss kills them both, a former shut-in scientist is reincarnated with his memories in a sword and magic world but must stop his former boss from finding a way to repeat the deadly experiment that could destroy the world."