Stephen the Little Pretended to Be Tsar Peter III
When Scepan Mali arrived in Montenegro in 1766, it was. He came from nowhere and had no history that has been recorded for posterity. Although he claimed to be Tsar Peter III, his claim to fame was that he was a herbalist and adept healer. He didn't seem to be Russian, which was noteworthy, but more crucially, Tsar Peter III had passed away in 1762.
He was, according to the tale, looking for safety from his adversaries. It appears that, at least in public, he never actually asserted his claim to be the Tsar. Instead, stories started to circulate, which he refused to refute while also making indications that they might be genuine, until it seemed like everyone was aware of the story.
He was proclaimed monarch of Montenegro by the assembly, which was an unexpected turn of events, and ruled the state for the following six years. Even the Russians, who had dispatched envoys to remove him, later endorsed him as leader since he appeared to be capable of handling the position. His authority only came to an end when Ottoman soldiers paid a servant to kill him.