This is a SMB1 hack with a SMB3/SMW graphical style. There are three challenging sections to complete to get through Iggy’s domain and almost everything has been changed in one way or another. Difficulty is about that of the latter half of the original SMB1.
The dreaded IGGY KOOPA has had enough - his father has shown time and time again that his method of kidnapping the Mushroom Kingdom’s princess (running in and out and hoping for the best) is useless… but he just won’t stop trying. Iggy, instead, decided to take matters into his own hands. Calling on dark Koopa magic even his siblings aren’t aware of, he was able to take command of a hidden away “Dark Realm” deep within the Koopa Kingdom, and used these evil powers to brainwash a vast number of his father’s army into serving him alone. Now he just sits back and schemes, waiting for the perfect moment to invade the Mushroom Kingdom and take it for his own. The news of the impending invasion reached the Mushroom Kingdom quicker than he anticipated, however. Defecting Koopas warned the Mushroom royalty of Iggy’s imminent approach, and with no one else to turn to, they called on Mario for help. He, of course, agreed, and so set off into the Dark Realm to kill Iggy Koopa.