To enter the Blighted Grove you must have a Bone Machete in your backpack. This item can be crafted by Blacksmiths who have learned the recipe from Jamal the Fisherman, who can be found outside the dungeon.

To gain access to the Peerless boss, Lady Melisande, you must complete a quest from the Frightened Dryad found in the dungeon. This quest involves kiling 12 Insane Dryads and the named Harpy Saliva. The reward from the quest can be dropped into the basket next to the dryad, which will result in a number of teleport items that will take you to the peerless area.