Ardyn takes advantage of Ignis' blindness to mess with him. Ignis knows at least some of it is Ardyn but he can't prove it, and soon gets to the point of doubting what's Ardyn and what's himself.

Anything from non-conning him in another bro's body to moving his things around so he can't find them.

Would love it if certain things were set up that meant Gladio and Prompto doubted him as well.