Complex sentence: Mickey encounters DiZ, who reveals himself to be Ansem the Wise and uses a device that dissipates some of Kingdom Hearts' power, but a system overload causes it to self-destruct, both engulfing Ansem and miraculously returning Riku to his original form.
Split into simpler sentences: Mickey encounters DiZ, who reveals himself to be Ansem the Wise. Ansem uses a device that dissipates some of Kingdom Hearts' power, but a system overload causes the device to self-destruct, both engulfing Ansem and miraculously returning Riku to his original form.