To enter the Twisted Weald you must be on the quest to kill Dread Horn, which is given by one of the elves in Heartwood or Sanctuary. If you have already completed that quest you will still have access.

To gain access to the Peerless boss, Dread Horn, you must gather a number of items from the creatures in the dungeon and drop then on the small statue in the ring of stones in the swamp area. This will result in a number of teleport items that will take you to the peerless area.