Transformation into Passive Voice:
The prisoners had been taken to the cell by the soldiers.

Explanation:
When transforming a sentence into passive voice, the object of the active sentence becomes the subject of the passive sentence, and the subject of the active sentence either becomes the agent in the passive sentence or is omitted altogether.
In this case, "the soldiers" (subject) in the active sentence becomes "by the soldiers" (agent) in the passive sentence. "Them" (object) in the active sentence becomes the subject "prisoners" in the passive sentence. The verb "had been taken" remains the same but the form changes to fit the passive voice structure.
The passive voice is often used when the focus is on the action and not on the doer of the action. It is also used when the doer of the action is unknown or less important than the action itself.
By using the passive voice in this sentence, the emphasis is on the prisoners being taken to the cell by the soldiers, rather than on the soldiers performing the action.