The giant walking barge you start DOOM Eternal in, on earth. This was a really gameplay idea to give some cool immersion and scale at the very beginning of the single player campaign by having the first part of the first level take place inside this structure on the back of a MASSIVE Titan Demon (credits below for demon).
After early ideation, I started with posing the demon and blocking out the support structure and basic barge structure before taking it to completion.
Titan Demon:
Concept by Jon Lane,
Modeling by Peter Boehem
Additional Surfacing by Phillip Bailey
Background Painting here:
https://www.artstation.com/artwork/eaQXGw