“Chilling doodle, and just gotta get one of many ideas out of my system.  
As someone who greatly values life, Celestia refuses to use her true power. Simply activating it will cause the atmosphere and the water to dry up. It will eradicate anything it touches into nothingness. Its sheer heat can burn everything around her to ashes. If she ever uses it for too long, it will destroy her foe, Cantelot, the entire Equestria, and eventually herself.  
Shout out to Bleach.”
 
Naturally, this means that Tirek will steal her power and that Twilight’s zanpakuto spirit looks like a young Tirek because reasons.