Note: You can only obtain the key once. If you get it during the Hungry Deserter, it will not be on the prisoner's corpse in Return to Ostagar.
It unlocks a chest that is guarded by a Tranquil during the daytime near the Circle of Magi camp during the Battle of Ostagar. During Return to Ostagar the chest is located slightly northwest of "Elric's buried key" on the map.
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