Somewhere in the land of Ardyl you will find a camp with a lady, Genna, bedridden by a sickness, and in the tent next to hers will be a chest with a scroll from Ger the Sorcerer to Gutrick, a jealous brother in law. Show him the letter and he will see the error of his ways, and follow you to Ger to retrieve the antidote. However, Ger doesn't have an antidote ready, so he gives you a cup with instructions to go to the fountain of light and fill it up there.

On levels 5-8 you will find 4 prisms, which you need to place on the outside pedestals at the fountain of light on level 6. This will create a beam of light bouncing from one prism to another, ending at the middle pedestal. That's where you need to place the cup from Ger, and it fills with the magical light, thus creating the antidote. Take this back to the Genna, who will drink deeply of the antidote, and then entrust you with the Crystal of Virtue.