The Old Wandmaker asks the Hero/ine to help him find a Seed of Rotberry. The Rotberry seed will be in its plant form (Rotberry Shrub), at the same depth the Old Wandmaker is in.
Upon trampling on (or triggering) the shrub, it will turn into the Rotberry seed, apply the rooted debuff on the Hero/ine (if stepped on), & release toxic gas. When picked up, the seed will emit a challenge sound (i.e. Scroll of Challenge), alerting enemies at the depth with you, to the location the seed emitted the challenge sound from.
Oh, what a pleasant surprise to meet a decent person in such a place! I came here for a rare ingredient - a Rotberry seed. Being a magic user, I'm quite able to defend myself against local monsters, but I'm getting lost in no time, it's very embarrassing. Probably you could help me? I would be happy to pay for your service with one of my best wands.
Upon speaking with the Old wandmaker without the Rotberry seed: Edit
Any luck with the Rotberry seed, <insert class>? No? Don't worry I'm not in a hurry.
Upon returning to the Old wandmaker with the Rotberry seed: Edit
Oh, I see you have succeeded! I do hope it hasn't troubled you too much. As I promised, you can choose one of my high quality wands.