The Potion of Wisdom is found at Geshniz's Headquarters in Broken Valley, in the possession of Black Ring General Geshniz (she must be killed to obtain it). Her headquarters are located through a teleporter next to Maxos Temple, to reach it you'll need to use dragon form and destroy all the nests and ballista towers in the south and west/northwest.
You can use the Potion of Wisdom to permanently gain 5 points of Intelligence. However, this will use up the potion and prevents you from completing the quest, so it's only a good idea if you chose Barbatos as your alchemist. Once you have the potion, return to the Battle Tower and give it to Allan.