I got the GOG version of the "DKS Developer's Cut"
In the game I have unlocked all achievements except for 1 "secret achievement"
I went through the list on:
http://divinity.wikia.com/wiki/AchievementsUntil I found an achievement I did not have and figured that was the one. That one achievement is "I trained a pet".
(what would training a pet have to do with resurrecting Ygerna?).
Ygerna explains she arranged her own resurrection by training a pet, you the dragon knight are her pet. She uses those EXACT words and the description of "I trained a pet" says "resurrect ygerna"... seems pretty straight forward to me.
I do however have the "dragon knight" achievement. and in fact looking around there might be a few other different achievements. For example I do not seem to have "Adrift" as well.
Do you know of a correct list of all achievements for the PC D2:DC version? That way I could go through it and find out which achievement I am actually missing