I can solve this puzzle, "Unknown people" means "Unknown thieves", since it’s Warrab state then thieves identity likely to be Dinka since majority of Warrab population is Dinka, Nuer thieves cannot be excluded either because they can come from nearby state and steal Dinka cows!
This type of business "cattle raiding" is going on between Dinka & Nuer, Dinka-Dinka
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