Wagon Down is a Side Quest in Sword Coast Legends. You can start this quest by William Ray in The High Road
- Look for Sarah
- Inspect the horse
- Bring the bridle back to William
Bone-Strewn Burrow. Inside the cave is the mushrooms you need. Kill the Spiders, and head to the very back of the cave where you will spot a horse amidst the webs. Interacting with it will reveal Sarah's bridle and a Giant Spider and smaller ones will ambush you. Defeat them and head back to William.