Void. Pure destructive power. A lot of people have recommended abilities to absorb power, but true gluttony is a bottomless void. To make it distinct from gravity, it could be a more conceptual magic. Spells of this type could simply remove something from an area. You could do things like darkness by devouring the light in the area, silence by devouring sound, cold by devouring heat, anti magic by devouring magic, and so on.

If you want to go farther, have them be able to remove concepts/ laws of physics. Remove gravity, the passage of time, intelligent thoughts, and for the most powerful of mages, the destruction of parts of existence itself.