The in-game dream -- Archdemon defeated, Duncan alive and well, safe in company with other Grey Wardens -- is actually not a bad choice for Kasia, especially the bit where Duncan is back. At that point in her life, it could really only have been better if Leske and Rica were also there, filled with adoration thanks to her heroism.
As for a nightmare, it would have to be waking up and finding herself back in that cell in Beraht's dungeon, except this time with no chance for escape. To know that she could save neither herself nor Leske, and to know that Rica would be left to Beraht's tender mercies, is the worst situation she can imagine -- made even more terrifying by the fact that it came all too close to becoming her reality.
This entry is also posted at http://owlmoose.dreamwidth.org/624935.html. There are currently comments on DW.