World's End Harem: Fantasia Vol. 10 (manga)

In the aftermath of the Reptile War, Arc emerged victorious over Arges, and the power of his Macht grew ever more powerful. Far away, in the City of Flowers, an aspiring mage named Retta is about to receive a similar power! Retta is devastated when his father is arrested for treason against the Empire, but as a low-rank mage, he feels powerless. That's when he meets Olga, a sexy demon woman with an offer too tempting to pass up! What Retta doesn't know is that Olga's in league with Lati, the Dark Elf who gave Arc his powers.

What are they scheming, and how does Retta fit into their plans?