Doom: The Dark Ages Hellbreaker secret and collectible
While the first mech mission in Doom: The Dark Ages had no secrets whatsoever, the second has just one. You'd think this would make the Hellbreaker secret and collectible dead easy to find, but you'd be mistaken.
The Hellbreaker mission only has one secret and one collectible, but they're actually the same item, so you're really only looking for one treasure: a toy. You'll get 85 gold and one armour Demonic Essence just for completing the mission, but you'll need the toy for 100% completion.