Good catch.
yv0751, it looks like a small bug. The code is in 200_support/220_dialogue.rpy:
Python:
elif ID == "QID_GWYNN_DEMONOLOGY" and mode == "on_complete":
call achievement_notify(gwynn, "demon mistress") from _call_achievement_notify_3
$ nyx.achievements.append("demon mistress")
return()
IMHO, it should be
gwynn.achievements.append("demon mistress") instead.
EDIT: And to answer your question,
Test279, this is only awarded after finishing the QID_GWYNN_DEMONOLOGY quest. The quest itself doesn't seem to be defined, however, so it's either an omission or a placeholder for a future quest.