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.
If you think the Fell Thunderbrute Head sounds like an item dropped from a boss, you're right—sort of. While you'll need to defeat the Fell Thunderbrute to retrieve its head, it's more like an elite version of the other enemies you've been fighting in the Shroud, so you need to be prepared when facing him. First though, his location.
You'll need to deal with enemies as you make your way down into the depths of the structure, so make sure you're prepared for a fight. Once you reach the bottom, you should spot the red-glowing Shroud Root easily enough. As you approach it, the Fell Thunderbrute will make an appearance.
Thankfully, he's not too difficult to deal with. He uses a two-handed axe so his attacks are relatively slow. You should have enough time to dodge and get behind him to deal damage, otherwise, kiting him around and picking him off with arrows also works. Once he's been defeated, loot his body for his epic axe, The Guillotine, and the all-important Fell Thunderbrute Head item.
Destroy the Shroud Root with your axe while you are here to complete the area and obtain a skill point. Now you can head back to your base with your reward.