On this page of the Assassin's Creed Valhalla guide you'll find a detailed walkthrough for the Miracle side quest.
Rewards: 1,200 XP.
In the place marked on the above picture (by the tree), you will meet an old man who wants to travel to Clee Hill to find a cure for his illness. Unfortunately, the man has a limp, so he won't be able to get there on his own.
Pick up the man and carry him up the hill. While you're there, place him in the stream.
Then proceed to the location shown in the image above. Near the ruins you will find a blind man who needs to be led to the same stream.
After a short conversation, the men are sure that the spring has no magical properties, but they will get along with each other and help each other. The mission will come to an end.