This story takes place not long after Athy(Nightmare) ate the black apple.
By eating the apple, he no longer hides his suppressed desires, his desire to monopolize his Citro(Dream). His horrible nature is revealed, That's why I named this route "True."
He continues to interpret his brother's words in a way that suits him, restricting his brother's actions and taking the best outcome for himself.
Citro wants to stop him, but it doesn't work because he has lost his power, so he has no choice but to follow him. And so the two of them will live under the red sky for the rest of their lives.