It has been a while since William's sword has made an appearance so let me refresh the memory.
William wields a Hunger Null Sword. Inside the hilt is a sentient black hole which William defeated and made a compact with. At rest, the hilt is just that. A hilt. When activated, the black hole emerges in the form of a blade, surrounded by a quantum field. When in this state, the sword functions much like a normal, though strong, blade, with the stabbing and the cutting.
When the quantum field is turned off, the black hole begins eating. Willpower is required on William's part to constrain the black hole to eating just his opponent and not, say, the entire world.
William can only disengage the quantum field once per combat. In D&D 4E terms, this is an encounter power. He's reluctant to use the ability when he knows he has a long day ahead of him because the contest of wills to reign the black hole in can be draining.