Being weakened by being sealed for so long, Apep took this form after crating a contract with Corvo, allowing himself to be used as a tool for the young sorcerer while in turn feeding of his magic to restore his own. His wooden form representing his weakened state while the metal represents when he is almost restored to full power.

I'm curently working on a story to better explain his current state.

art:

Apep/Apep's staff: