There is a link at the top of the inventory files post to a step-by-step description which was written for Divine Divinity, but there are no significant differences in Beyond Divinity.

IIRC = If I Recall Correctly
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Ideally the prisoner would take any water (or any bottle of water), or at least the special bottle would be non-drinkable. Maybe the prisoner should have been poisoned, or something (by a trap), and asking for a cure potion, in the form of a herb powder / extract that imps use when cooking.