Hey! I was wondering if anyone here knows exactly how to find all the items in Sons of Horus? I am only missing one collectible item, which should be in the secret tunnel under the temple (where the alien God is chained) but for as much as I search the area, I cannot find anything (the sonar seems to be picking up something in the antechamber, but it is just the sarcophagus I opened to obtain the heart).

In those lines, I noticed there is an Achievement for "Freeing the unknown prisoner". Any ideas how to obtain that one? (I thought it would be by touching the chained alien God, but that didn't turn out so well).

Thanks in advance.