In a bid to defeat the Gate Guardian, Yugi uses Polymerization to fuse his Summoned Skull to Joey's Red Eyes Black Dragon, creating the Black Skull Dragon. Through the combined effort of their unified teamwork, Yugi and Joey defeat the identical Paradox Twins, winning enough star chips to allow them inside Pegasus' castle. Also, Yugi and his friends manage to escape the caves after solving a riddle posed to them by the Twins. Meanwhile, Kaiba is able to find Mokuba from within his search, but Pegasus suddenly appears and uses his Millennium Eye to steal Mokuba's soul. After doing this, Pegasus then gives Kaiba an ultimatum: Kaiba must first defeat Yugi in a duel so that the former can gain the right to face Pegasus himself in an attempt to win back his brother's soul.