The Archdemon or Dark Prince is a very powerful boss in Dark Castle. It is one of the bosses which have a second form.
The Archdemon is from the game Seiken Densetsu 3, a sequel of the well-known SNES title Secret of Mana, only released in Japan. The Archdemon originally was the ruler of the ancient kingdom of Lorimar, Stroud. He was called the Dark Prince by servants because he tried to kill his brother. He made a deal with the ruler of the Underworld. He would become the new ruler of the Underworld in exchange for destroying his own kingdom. After doing this, he (seemingly) killed the old ruler, and so became the new ruler. Now he has set his eyes on the legendary, all-powerful Mana Tree, Giver of Life and Power.
The Archdemon has two forms. In the first, he attacks with many moves, and he changes color every time he is going to do a new attack. Mario has to throw blocks, that grow from the boss his attacks, against him.
In his second form, he grows even more powerful. Now, Mario has to do a Spin Jump when the enormous sword of the Demon comes out of the floor. He will bounce off the sword, and land on the boss' head.