He was attempting to create a death ray. Incomplete diary entries were found and Russians and Germans both tried to create one using his incomplete notes. It's not a myth, since he stated in the journals what he was doing. Unfortunately by that time he was also batshit insane.

Geniuses who either go insane or were pretty much insane in their adult lives intrigues me. Pythagoras created his own religion loosely based on Plato teaching but he thought beans were evil (seriously), Lord Byron was a widely known pansexual pedophile and kept a bear in his dorm room (Idk if that is insane or pure bad ass). But most interesting to me is Mark Twain. His last written piece which he specifically did not publish involved a man who lived alone, isolated from society. One day a man who can perform magic visits him and opens his mind about mystical things religious in nature. Eventually the stranger reveals he is an angel. But then, oddly, at the end he informs the man that the angel is not real. The man, the angel says, is also not real but only a figment of someone else's imagination. He is simply an idea. Mark Twain wrote and rewrote this story 3 times but never properly finished it ever, the most complete finish just as the stranger simply leaving after crushing the man's sense of reality. Mark Twain was known to be deeply depressed at this time and some wonder if he tampered in magic (his house is reported haunted because of this). It seems he was convinced that he was just a pawn in someone's game, just an idea in another genius's head. A renowned atheist, even Twain could not embrace the finality of life in his last piece that he never personally published.