This page of our AC Valhalla Dawn of Ragnarok guide shows the locations of all gear items appearing in Eitri region.
Fire Giant Cloak
Location: Eastern part of the city, right next to the border with Svaladal.
This chest is hidden in the building. You're going to need the Power of Muspelheim, as the entire floor is covered in lava.
You have to move the stone ledge so that you can climb it to get to the nearby ladder. You should see the chest immediately after climbing the ladder. No key is required in order to open it.