The step says "Find and speak to Squire Rustbucket near the mill by the brook," but there is no Squire Rustbucket. Instead, examine the banner outside Bog Bottom Mill ( 10, -1, -605 ) /waypoint 10, -1, -605. (CAREFUL: there is a heroic group of goblins near the banner)
Return to Sir Tatters.
Examine a chest in the Boar room in Runnyeye ( -74, 1, -91 ) /waypoint -74.02, 1.26, -91.28.
Obtain the key for the chest from enslaved warboars. There are some in the room unless you already killed them, and more in the spiral passage near the entrance e.g. ( -84, -27, 118 ) /waypoint -84, -27, 118.