The nature of the conflict:
It is not good versus evil in the sense of two armies clashing (though it can be depicted that way). It is truth versus illusion. The Dark Lord's power is the power of the lie — the lie that you are not the Son, that you are alone, that you are guilty, that the Kingdom is lost, that there is no way home.
“the children of the lie”
The Prince defeats the Dark Lord not by superior force but by seeing through the lie. The moment he truly knows who he is, the Dark Lord has no power over him. The battle is won not by killing but by waking up.
This is why the crucifixion is the victory. Jesus "defeats" evil not by destroying it but by demonstrating that it has no power over the one who knows the truth. Death could not hold him because death is part of the lie.