The Nine showing interest in him makes me think he is at least a part of it. You don't live that long through a struggle without some secrets, or more than we know. We used Rhulk's darkness/attacks to get him to a weakened state, so I think harnessing Darkness will be pivotal against The Witness. Possibly just one of the first Light/Dark line-walkers.

TANIKS, THE ESCAPE KEY, THIRSTS FOR YOUR CLOSURE!