Dream's Deep is mainly a dark location with many images of Luigi around.
taken Princess Peach down here. When Mario first enters the dark dimension, he notices that Dreamy Luigi isn't around but after noticing all of the images of Luigi and messages around, Prince Dreambert concludes that the Dream's Deep is Dreamy Luigi himself. After a little more exploring, Dreamy Luigi decides to join Mario's side and they press on. Finally, Mario, Dreamy Luigi, and Prince Dreambert finds Antasma with Princess Peach but Bowser suddenly appears. Antasma manages to convince Bowser to join him in a fight and the groups square off. Mario's group is victorious but Antasma gives Bowser a little more power to knock out everyone. Antasma then escapes the Dream's Deep with Bowser saying that they will capture the Dream Stone. Mario soon wakes up and he and Prince Dreambert leave Dream's Deep.
Dream's Deep is visited a second time in order to help locate the ancient Zeekeeper to help break the barrier around Antasma and Bowser's castle. Mario and Dreamy Luigi do so, and learn two new moves, and they group learn from the statue that they need to assemble the five pieces of the Ultibed and then have it assembled by a Bedsmith. Mario and the rest leave Dream's Deep in order to find the five pieces of the Ultibed.