Deathstalker Mantle not working

[ShadowRose99]

I can't find any answers to this anywhere. I have yet to turn invisible after killing someone, and I don't know how to fix it.
- I have cull the weak turned off
- I am a sorcerer, and have tried both with ray of frost and up close with a dagger
- I am not dual-wielding
- I have tried being in "hiding/sneak" mode
- I have unequipped, restarted the game, then reequipped

The only thing I can think of is that I'm in the goblin camp and trying not to fight 10 at once, so I'm trying to sneak up and kill them, therefore I'm not in an actual fight. Does it only work inside combat? Cause that's honestly really frustrating if so. Or am I missing something?

[Kernest]

I think it probably works only inside combat, but even if it didn't, the invisibility it grants is 2 turns, which is not very long outside of combat.