The map is procedurally generated. Map layout, Number of Secrets and Secrets Locations will vary for each playthrough. One Secret Rune can be collected only after you have completed the entire main campaign. So, in your first playthrough you may not be able to collect all the secrets. Check the video at the end of the page if the written walkthrough is not clear.
It is just next to the forge entrance.
As you move ahead inside the forge look around the corners for these chests
In the area where you get the objective to overload 8 cores you’ll find these secrets on small cross-over bridges. Interact with the pillar on the bridge to start a fight. A secret chest will be revealed as the fight ends.
In the area where you get the objective to overload 8 cores you’ll find these secrets inside a crypt. Enter the crypt and interact with the pillar to start a fight. A secret chest will be revealed as the fight ends.
Secret 8 (Can be collected only after completion of Main Campaign)
Just after entering the forge, right in the corner in the first area you’ll find a switch. Interact with the switch to open a passage. Go inside and collect a Secret Rune. Collecting 9 Runes will unlock a Secret Cow Mission.