If he vanished in 398, then that means Jagar Tharn was still impersonating the Emperor. Perhaps Calaxes knew more than has been let on.
I doubt in 398, to be honest.
It cames from the "http://uesp.net/wiki/Lore:Words_and_Philosophy", where it's dates 498, which is obviously a typo; so it was fixed to 398.
But according the book Lady Benoch began to fight only two years earlier (in 396). No way she cold become elite Emperor's guard for such short time. So I think there was typo in the second digit, not in the first. Therefore 408, or even 418, and not 398.