For this, you need to get one of the attribute scores of your main character to 25 or above.
An easy way to do this, is to drink Potions of Mind Focusing, which gives you +3 INT and DEX. The effect stacks, so you can keep drinking potions until you reach 25 in either INT or DEX.
The potion can be bought at the Nashkel Carneval (Merchant located at x 1865 y 3518), most temples and in many general stores.