Complex sentence: Furthermore, Terra is facing extinction; not only is the planet growing old, but the knife-headed monster, which the Terrans call "Guiron," is their last defense against the Space Gyaos.
Split into simpler sentences: Furthermore, Terra is facing extinction; not only is the planet growing old. The knife-headed monster, which the Terrans call "Guiron," is their last defense against the Space Gyaos.