Complex sentence: No one realizes that the "mummy" is actually roaming around campus busily tracking down the students who possess the stolen crystals, because the crystals are actually crucial components of an intergalactic transportation device that will allow the alien to return to its home planet.
Split into simpler sentences: No one realizes that the "mummy" is actually roaming around campus busily tracking down the students who possess the stolen crystals. The crystals are actually crucial components of an intergalactic transportation device that will allow the alien to return to its home planet.