Complex sentence: The doctor appears on the computer screen, calling himself the creator of a new world and ordering the Slisaurs to finish off the team; however, the Slisaurs fail in their attempt, and the heroes moves deeper in the basement.
Split into simpler sentences: The doctor appears on the computer screen, calling himself the creator of a new world and ordering the Slisaurs to finish off the protagonists. But the Slisaurs fail in their attempt and the team moves ahead, deeper in the basement.