Necron exploded after being defeated by the party. He isn't eternal, and it's not a super powerful cosmic being. He fought, and lost. The End.

And what he said(about being eternal), contradicts what we see in the game itself. But that's not the first time this happens in FFIX. Soul Cage said he foreshadowed his own dead, and it wouldn't be by the hands of Zidane and his friends. But they killed him anyway.

Will you guys ignore Soul Cage's death and state that Zidane and the party didn't kill him, only because of what he said before the battle? :rolleyes2
This is what you all are doing in relation to Necron.