There is no direct evidence of any Guardian becoming an agent of Darkness. However, it is heavily indicated that Dredgen Yor might have actually come to do the Darkness' bidding. When the Hive bones Rezyl grafted into Thorn began to taint him with Darkness, he turned his back on the Light and ceased to be a Guardian. According to the perspective of the Shadows of Yor, there is actually a sliver of a chance that Dredgen wasn't working specifically for the Darkness despite his use of some of its practice and hostility towards the ways of the Light. So Dredgen is a maybe Also, according to the few stories we have, the Ahamkara would tell Guardians things that made them doubt the Light, causing many to turn their backs on the Traveler. However, it is highly doubtful that any came to serve the Darkness Toland the Shattered studied the ways of the Darkness and came to be practiced in some of its ways, and Eris through his journals also became like that, but both have proven to be fully supporting of the survival of the Light and destruction of the Darkness. It's one of those "to kill it you must understand it" philosophies As for Oryx's lines, he is trying to Take you. Taking works by converting your will to serve Oryx and the Darkness, so naturally he says things about sharing the Darkness with you and "let my will set you free" (direct quote) as that is how Taking works in his eyes On the note of "The Darkness has consumed you" line, that just means you died in an area rank with Darkness so your Light was snuffed