Players can enter the Deep Dungeon with any combination of jobs and a group of any size between 1-4 players, earning experience and special weapons for use in the outside world as you level from the start once again. Items include tools to revive other party members, improve defenses, heal yourself and others, and even turn into monsters to clear difficult sections. The Deep Dungeon is slated for release with patch 3.35, but you can get some of its flavor by checking out the development diary.
Source: Official Site