Lucius the Eternal from Warhammer 40000 comes to mind.
Yes, he might be a genetically enhanced supersoldier turned daemonic Chaos champion. Yes, he might be one of the most skilled swordsmen in the universe. But this is not what makes him undefeatable.
Lucius happens to be a champion of Slaanesh - the Chaos god of excess, lust and pride - and Slaanesh is rather fond of his servant. So the Chaos entity created a curse that afflicts anyone who manages to kill Lucius in a duel. Anyone who slays the daemonic swordsman in a duel and feels pride in their achievement becomes possesed by their fallen foe’s spirit. Soon their body is ravaged by scars, decay and painful mutations until it falls apart, revealing Lucius, reborn and ready for a new challenger.
There might be a way of preventing his rebirth by self discipline but no one, human or alien, managed to do that in 10000 years.