Apparently, killing monster is not a conditionto open it. I’ve had a long fight with them (mostly, trolls) several times, and the chest became available while the guardian was still alive.
Steps to reproduce:
- Play Chaos Wastes.
- Open the chest and get some tough monster.
- Take your time.
Here’s the video of me fighting grudge marked rat ogre for quite some time. The chest opened while long before the fight was over:
In addition, here are console logs, if it helps the matter:
console-2021-11-05-11.31.16-321a934f-9c34-43be-afa7-1bfe88e09d2a.log (2.8 MB)