If Fight to the Death is cast before blockers are declared or after combat ends, it won’t do anything.

A “blocked creature” is an attacking creature that has been blocked by a creature this combat, or has become blocked as the result of a spell or ability this combat. Unless the attacking creature leaves combat, it continues to be a blocked creature through the end of combat step, even if the creature or creatures that blocked it are no longer on the battlefield or have otherwise left combat by then.

A “blocking creature” is one that has been declared as a blocker this combat, or one that was put onto the battlefield blocking this combat. Unless that creature leaves combat, it continues to be a blocking creature through the end of combat step, even if the creature or creatures that it was blocking are no longer on the battlefield or have otherwise left combat by then.

Attacking creatures that haven’t been blocked are unaffected by Fight to the Death.