How to get a dragon soul for Bend Will?
I had Bend Will, so I tried to unlock it because I have 22 dragon souls but it wouldn't let me unlock it. I just went and killed a dragon but Miraak stole the soul from me. What do I do now?
You need to go through the main quest line of the dragonborn dlc to be able to unlock it. Once unlocked you can spend a dragon soul on it by clicking 'unlock' or pressing the according key. This page on the site covers that extensively and refers to other helpful pages.