Instructions
Order and install the downloadable content "Wizard's Tower" and then journey to Frostcrag Spire. You'll find a bottle in the vault amongst a bounty of other loot.
Travel northeast of Bravil until you come across Fort Aurus. Explore the fort until you find a room with an adept and some alchemical equipment. You'll find two bottles of ShadowBanish in the chest there.
Go to Niben Bay and swim to the island fort, Fort Grief. Climb the wall. Just beyond it, you'll find a chest containing another bottle of ShadowBanish.
Travel southeast of Bravil until you find Fort Irony. Enter the fort and continue down the main hall until there's a door to both your left and right. Open the left door and then loot the chest by the cooked rat for another bottle. This is all the wine you need to satisfy the innkeeper's request and get your reward.