Nel has an issue that she needs your help with. Tenakth raiders have been attacking the settlement of Riverhymn and she wants you to help put an end to it. Unfortunately, none of the residents of Riverhymn really know how to fight. You’re gonna have to work hard on this one. Here is how you complete The Roots That Bind side quest in Horizon Forbidden West.
After speaking to Nel in Plainsong after you complete the Death’s Door mission, you will be directed to Riverhymn. This northern settlement is up near the top of the map above Plainsong. Make your way up there following the path that the game shows you. Once you get close to Riverhymn, you will see a group of raiders attacking the people of the settlement. Help them out.
Once all of the raiders are killed, you will need to follow Kue. Follow him into the camp and you will get a cutscene explaining that you will need to go to the top of the mountain and retrieve materials from the Widemaws so he can make weapons. After the cutscene, follow Kue to the base of the mountain. You will need to start climbing.
Make your way up through the mountain. The path is pretty straightforward. Use your Focus to find the handholds in the rocks to progress forward. When you reach the main chamber, you will need to dive underwater and follow the current to progress forward at first.
When you reach the top of the mountain, you will see two Widemaws in the area. You have two options here. You can either sneak behind one of them and collect the canisters they drop or face them head-on. Be careful when taking them on. Remember that Widemaws can be heavily damaged by shooting the vacuum mechanism inside their mouth. When you have all six canisters, return to Kue and get ready for one final fight against a group of Tenakth.