2778.255 -- The following events all pre-date the changeover from Heidelmere to Highguard:

- Arista & Garthain came to Heidelmere
- destruction of Ridena (a few years later)
- the start of civil war in the inworlds
- the split between Vissarion & Ambros

Arista's departure from Ridena is described as a choice rather than of necessity, but not beyond that. (in The History of Highguard)

Arista was buried in an unmarked grave in the royal crypt. I've identified three which it could be, but they are not easily openable.