The Restless King - Act 5 Side Quest
Start Location: King Gylfi's Settlement
Quest Location: Barrowlands/Great Barrow
Objective: Obtain the Ancient Breastplate from the Great Barrow
The Restless King is a Side Quest which begins by speaking with Etheltrud in King Gylfi's Settlement (Scandia Portal). For this quest you will need to venture into the Barrowlands, an area northeast of town and retrieve an Ancient Breastplate. While you're obtaining this item it's recommended you grab the item we need for The Magic Cauldron Side Quest nearby as well.
You can find the Ancient Breastplate we need inside of one of the Great Barrows the central portion of the Barrowlands (pictured below). After you open the chest that contains this item and pick up the Ancient Breastplate an Ancient King undead quest enemy will spawn. You don't have to defeat him if you do not want to, all you need is the breastplate to complete the quest.
Return to Etheltrud in King Gylfi's Settlement once you've obtained the Ancient Breastplate and give it to him to complete the quest.