The Fell Thunderbrute Head is one of the items you need to increase the Flame level in Enshrouded. Doing so expands the maximum number of Flame Altars you can have active, giving you a larger fast travel network. It also allows you to spend even longer in Shrouded areas, so it's well-worth gathering the materials needed for the upgrade.
You won't need the Fell Thunderbrute Head until you're ready to increase the Flame to level three. The materials for the first upgrade are easy to get hold of, but when you go to level it up further, you'll see the head listed, along with other, rarer resources. For now though, here's where to get the Fell Thunderbrute Head so you can increase the Flame level.
