Complex sentence: In order to cover up Chiharu's death, the executives at Tristar Promotions disposed of her body in the city sewers, where the "nano-bots" in Chiharu's body malfunctioned as they revived her into a sub-human monstrocity.
Split into simpler sentences: In order to cover up Chiharu's death, the executives at Tristar Promotions disposed of her body in the city sewers. It was there that the "nano-bots" in Chiharu's body mutated, and eventually resurrected her as a sub-human monstrocity.