there are many theories and arguments about who and what class dredgen was before he was lost to the darkness, but none (that i have seen) concerning [b]why we don't know[/b]. within the current lore the only guardian to fall to darkness is dredgen. if he truly is the only guardian that has falllen to the darkness then the reason we don't know could be that the info was erased from our memories and any records such as ghost fragments (if you are wondering how there are some that believe that the ghosts have the abilities to erase our memories). the reason for erasing our memories of him could be to prevent other guardians from discriminating against other classes because one of their own was a servant to the darkness. it could also mean that more like him could have been erased from our memory entirely. Edit; I'd like it if you could post other explanations as to why it isn't in the lore