The DLC story takes place on an island to the north of the main story area, but players can also take those mechs back into the original campaign, to raise a little hell. The mechs can be upgraded, and they can be driven by AI allies, so that protagonist Rico Rodriguez can do other murdery stuff. Just Cause 3: Mech Land Assault costs $11.99.
https://www.youtube.com/watch?v=lZ5o3dJzTxU
Souce: Polygon
Well, fingers crossed for a real performance improvement patch on consoles, this DLC is looking quite nice and, as a season pass buyer (I know, I'm part of the problem) I'm hoping to play it.
Edit:
Oh, and it's out June 3rd for season pass holders and June 10th for everyone else.