The Phoenix Force. Going by on panel showings Knull is a skyfather level being at best whose opposite is the Enigma Force/Captain Universe.
Powerful, but way below the peaks of the likes of the Abstracts and the Phoenix Force.
It was specified that Knull isnt a fundamental force, he didnt create and doesnt embody the darkness, it preceded him, hes just an ancient being that learned to wield and shape it. Thats as opposed to the Abstract Oblivion who embodies the void/darkness. A pecking order has been established.