How did tuxedo mask die?

Tuxedo Mask, from the anime and manga series "Sailor Moon", did not die. He is actually the human alter ego of Mamoru Chiba, who is still alive and well at the end of the series.

It's possible you might be thinking of a fan theory or alternate ending, or perhaps you are confusing Tuxedo Mask with another character.

If you can provide more context about where you heard this information, I can help you figure out what you might be thinking of!