Relatively recent update made trolls vulnerable to electrical damage. The healing factor of Deep Trolls in the Skull Crags are still unaffected by electrical damage.
Steps to reproduce the Bug:
1. Lower the troll's HP enough to where you can fatally strike them
2. Utilize an electrical essence, imbue or spell to kill them, or
1. Constantly use an electrical essence or imbue to kill the troll
2. Attempt to fatally strike them while still using the essence or imbue
Expected Result:
The troll dies.
Actual Result:
They continue to live and heal.
Screenshot(s)
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Additional Information/Notes
1. Tried this using a spellsword electrical imbue and also use the Lightning Bolt spell
2. Don't remember if trolls have their own subtype, but the Deep Trolls are listed as giants
3. Have not tried to see if Fire will kill the trolls since the electrical-immunity update, but have killed the trolls before with fire