Dwarven rats.
Each time you speak to a dwarven miner there is a slight chance they'll offer to sell you a Fried Rat for a few gold (50 or 70, depending on where you are).
Each Fried Rat permanently adds 2 mana and rage, but drains 50 leadership.
You can repeatedly speak to the same miner to get multiple rats. You can manually click on the dwarf then the 'goodbye' conversation option, but it's a lot quicker to use the Escape key to dismiss the dialog.
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