This page of the Assassin's Creed Valhalla guide includes a detailed walkthrough description for the Dreamwalker Warrior side quest.
Award for completion: 1200 XP.
In the place marked on the image above you will meet a warrior sleeping on guard, who set up a makeshift camp right by the rocks.
When you want to wake him up, he will just get up and start sleepwalking. You can't stop or wake him up so just follow him.
At some point, the sleepwalker will approach the chasm, but by some miracle, he will be able to safely ride the rope to the other side. Talk to him to finish the event.