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I've got an item called 'Festule the Alchemist's potion', and it says it's specifically for ogres. But what do I do with it? Is it somehow related to a quest in the Ogre's Tower in the Druid Grove?

I think it's a bug that it's in the Temple of Helm.. You're supposed to get it from (surprise) Festule the Alchemist himself. Take Mazzy to the Slums district near the Coronet and you'll find the quest.