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One twist here could be the "confusion" surrounding who the "foe" actually is...perhaps there are rumors/assumptions of trolls, orcs, demons, etc (take your pick) and the PCs assume the worst when reports of half-eaten farmers reach their ears. This could lead to some surprises if the PCs prepare for horrid "monsters", but instead find perfectly "normal" appearing people. This would work best perhaps, if the cannibals are not caught "dining" in-situ. I can see the exchange between PCs and Cannibals taking place on the road. The cannobals have dined and now return to their villages. The PCs warn the cannibals (who appear quite normal it must be stressed, not as deranged bone-in-nose types) about "farmers being attacked" in the area. The cannibals thank the PCs for the warning, nod, and go on their way. Much later the PCs sheepishly realize what is going on.