Upon completing the necessary quests, I was awarded an item called [Eye of the Black Prince]. When this item is used, it adds 1 prismatic socket to a sha touched weapon of your choice.
Here's the catch : You only get ONE
As it stands right now, there is no known way to obtain a second one (unlike with the +500 stats gem, where you can just spend 10k for a new one).
So make sure you use yours very carefully when the time comes for you to obtain it. Don't make the same mistake as me and use it on an item that you can easily replace.
If somebody does find a way to get a new one, let me know.