Mittelborg: City of Mages (UP)

Quite simple you just need to spend 300 Aether during one Serenity, which is the time before starting the tempest.
Everthing seems to count, even buying potions, scrolls and artiacts.

Biggest issue is to amass 300 Aether which gets significantly easier with each life as you get more and more bonusses.

Be aware that the whole game is buggy and seems to treat your first 2 Lifes like a Tutorial without the ability to unlock achievements.
If you like me fulfill this condition during the totorial you need to delete your save, redo the Tutorial and do the condition again.