Complex sentence: The party may choose to fight the first wall or escape, but both fights must be won before a Demon Wall closes in on them, resulting with the party being crushed to death regardless of current hit points.
Split into simpler sentences: The party may choose to fight the first wall or escape, but both fights must be won before a Demon Wall closes in on them. If the party cannot succeed in doing so, they are crushed to death regardless of current hit points.