"The spell's master is powerful."

Translation: Sētero āeksio kostōba issa.

[UitoriCoru]

Why is it setero aeksio instead of seteroti aeksia?

[Joel__W]

It's a mistake, it should indeed be sētero āeksia.

[Dedalvs]

? The sentence is correct as listed: sētero is "spell's" and āeksio is "master". Āeksia is plural.

[ReaDickersonRose]

When is seteroti used?

[Dedalvs]

When it's "spells'"—that is, plural possessive.