During this side mission, William requests that the player brings him some Yarrow. Upon delivery, William tastes the Yarrow, claiming it to have medicinal qualities; he then gives the player a crafted remedy as a reward.
Following the mission, William can later be found other times as a random encounter in the wilderness, where he will request the player to collect more plants in exchange for other tonics. After a few encounters he tells the player that he is leaving and is never encountered again.
- Tall Trees, West Elizabeth - near Tanner's Reach
- Lonnie's Shack, Scarlett Meadows, Lemoyne
- Cholla Springs, New Austin - between Armadillo and Ridgewood Farm
- Grizzlies West, Ambarino - Northwest of the Mysterious Hill Home.
- Grizzlies West, Ambarino - Top of the Chelonian camp hill, northwest of Moonstone Pond (possibly only after completion of Chelonian Master event).
- The player will lose honor if they steal William's horse.