Honestly, the biggest part of this character concept could likely be simulated with a single aspect like "Oathbreaker: Power". It implies what it needs to regarding the character's background (he's broken one or more oaths sworn against his magical power), and can be easily compelled whenever he tries to do things that are otherwise potent expressions of that power. On the other side of the coin, the player might be able to invoke this aspect as a means of intimidating or deceiving some opponents ("Sure, I'll swear an oath not to kill you..."). Obviously, a middling (2-3) Conviction might help with this, but it need not be if the character still has a decent Conviction regarding anything BUT his magical ability.