He is rumored to have caused swarms of aggressive gnats to terrorize High Hall, and the villagers have since refused to trade with him because of it. Quintus suspects that revenge is now motivating him to cause trouble in High Hall, and you receive the quest The Runes of Wrath to take care of Mundus and the trolls that have been showing up around the village. Mundus shows disdain towards the villagers, calling them "backwards peasants," but is willing to compromise if a method of acquiring food can be found that doesn't require him to deal with them again.
Mundus prefers runes of earth magic and takes great offense if you suggest that they are weak, which can be used to taunt him into attacking you.Mindreading him reveals, "It's a good thing I painted that Friendship Rune on the portal, otherwise these trolls would be out of control!" This allows the option of removing the rune, providing a third possible solution to the Runes of Wrath quest.
Mundus' Journal can be found on his table, and provides a little bit of information about the source of his ability to summon trolls.