Music connection
Behemoth contributed music to Mortal Shell II, and Ben Sahar appeared in the reveal. This is a soundtrack connection rather than a gameplay encounter.
What this search does not mean
Do not look for a Behemoth boss health bar, arena, drop, or quest reward. Those gameplay details are not part of the documented music connection.
Related videos
Watch the route and combat use
The first guide starts muted. Scroll on and it stays in the lower-right corner on desktop; close it at any time.