Entering the Fungal Cave completes Thrundarr's second quest to proceed beyond the Animated Forest. This is unexpected behavior and has some undesirable effects because getting to the Fungal Cave and back is much easier than crossing the AF or braving DH. (It is approximately as easy to get to FC and back as it is to cross the AF the first time.) I breezed through the quest text, so maybe that's been updated, but the old quest text dictated that the end goal is the COC level below the AF (as that is the dungeon level accessible by both AF and DH).