Complex sentence: Each Remnant is "bound" to a specific person, who is the only one who can then use their power; powerful Remnants that remain unbound for too long have the potential to cause a "collapse" and spawn monsters.
Split into simpler sentences: Each Remnant is "bound" to a specific person, who can use the power. Powerful ones remain unbound for too long will have the potential to cause a "collapse" and spawn monsters.