Slavery seems to be legal in Torumekia.
During the Dorok Campaign, it is boasted that 10,000 slaves were captured while Kushana grumbled about no opportunities to take slaves in the Periphery Campaign [1].
Nausicaä notes that slavery is happening because of Population Decline and that Torumekia went to war "just to get more people" [2].
References[]
a See Page Conversions Between Editions to convert Deluxe Edition volume 1 (DE1) to other editions