Daredevil: End of Days opens amidst a brutal battle between Daredevil and Bullseye. Daredevil is on the losing end of the fight and says the single word "Mapone".
Bullseye flies into an uncontrollable rage and kills Daredevil.
During Ben Urich's quest to find out what "Mapone" meant, he is told
by The Punisher that Daredevil knew Bullseye would kill him for saying that word - that it was a one-way switch that there was no recovering from. Of course, Frank didn't actually know what "Mapone" meant (or was still unwilling to reveal it).
In the last pages of the series, we learn that
"Mapone" is the name of Daredevil 's daughter with Natasha Romanava.
But what about this fact/word would cause Bullseye to lose control and kill Daredevil?