Dyrnwyn is yours, Gwydion said, as it was meant to be.
Yet Arawn is slain, Taran replied. Evil is conquered and the blade's work done.
Evil conquered? said Gwydion. You have learned much, but learn this last and hardest of lessons. You have conquered only the enchantments of evil. That was the easiest of your tasks, only a beginning, not an ending. Do you believe evil itself so quickly overcome? Not so long as men still hate and slay each other, when greed and anger goad them. Against these even a flaming sword cannot prevail, but only that portion of good in all men's hearts whose flame can never be quenched.

Chapter 21 - The Chronicles of Prydain (1964–1968) - Book V : The High King (1968)