I think the Wardens were briefly mentioned at the end of Halo Cryptum as these super OG Forerunner constructs that were regarded with reversnce or something. One of them was actually Master Builder Faber's lawyer of sorts during his trial. Actually, as I read this segment again it is referred to as [i]the[/i] Warden. An extremely old construct (hinted at being over 25 thousand years old) that was the prison keeper and defended prisoners during their trials.