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.
So you're not scratching your head confused where on earth (or hell) a secret could be in such a tiny mission, below you'll find the Doom: The Dark Ages secrets and collectibles in Hellbreaker.
Doom: The Dark Ages Hellbreaker secret and collectible
The secret Atlan toy collectible is hiding in plain sight. Immediately after you leave the mech and you spawn in on its arm, turn around, and you'll find it on the head.
It's a very cheeky hiding spot, as I instinctively opened my map after leaving the mech and assumed the secret collectible would be hidden further up the path. Only on replaying the mission, clueless as to how I could miss a secret in such a bite-sized mission, did I think to turn around here.
Best Doom The Dark Ages weapons: Superior firepower
Best Doom The Dark Ages Sentinel Shrine upgrades: Gear up