You must find Arana and save her from the Watchers. Follow your quest marker to Arana and once you get there, you will be attacked by Watchers. Take out 4 of them and the attacks will stop. You may now speak to Arana by the tree.
Recover Arana's Spear
Aloy will ask Arana to return to Thok but you will also have to recover the spear she lost to a Scrapper. Scan the area for her tracks and follow them all the way to the Scrapper.
Aloy will have to destroy the Scrappers in the area and then recover the spear that is lodged into the side of the Scrapper. Take the spear and return to Thok to complete the quest.