Being a descendant of William Black, you have the blood of an Archon, whose blood is as pure as William Black's blood. You have gained the ability to use all three Hero traits: Strength, Skill, and Will.
When you do something related to one of the Hero Traits, the trait becomes more potent. If it's Strength, you become stronger, more vigorous, and faster. If it's Skill, you can become skilled at unnatural rates, become faster, and more stealthy. And finally, if it's Will, your magic reserves increase and you seemingly become more knowledgeable.
Magic, the act of William. With Will, you can do a variety of things, like casting a fireball, lightning, and blasting the area surrounding you with a wave of fire. Or if you want to have more physical spells, then you will find yourself being able to use things like a Physical shield, which surrounds you with a protective sphere of energy that will absorb all damage at the expense of Will.
There are many things Will can do, but note that some spells have a moral inclination, meaning that you cannot master and go further with some spells. For example, some spells like healing, would require you to be morally a good person, and draining the life out of someone will require you to be a morally terrible person.