Complex sentence: Down there the group finds an unaffected dog and some zombies, as michael states that nothing could of been down here or it would've eaten the dog, a legless zombie falls from the ceiling and attacks bart.
Split into simpler sentences: Down there the group finds an unaffected dog and some zombies. As Michael says that nothing could have been down there or it would have eaten the dog, a legless zombie falls from the ceiling and attacks Bart.