Doing it wrong
Resurrect Aerten 10 times.
At the very end of the game you will enter Fate's Arena. Here you will fight Aerten. She's not really difficult, but to earn this you will need to kill her 10 times without destroying the central artifact. Each time you kill her, she will resurrect and start attacking you.
She's not all that strong and takes very little to kill, you just need to keep attacking her until this unlocks.