I don't feel like going through a 15-20 minuet long video to find that info. Exactly what did the interviewee say about it?
Pretty much just that there was a pretender to the Tribunal. There may be more to it that he said but i dont wanna rewarch it either and im on a mobile device.
I'm assuming something like some figure pretending to be one of them or a fourth god being of some sort. Though I think it is likely some conflict with the Ashlanders and maybe some being that has managed to gather them together or something. I'm having a hard time envisioning the Tribunal struggling with such a person though.
It is also mentioned that the Akaviri invasion that brought the Nords and Dunmer together only happened 10-15 years prior to the setting of TESO.
I would recommend watching the video though. There is some good stuff in there and something about some Rancer War that brought the Daggerfall Covenant together.