The Necromancer's Stone; Rucuun's Theorem

"Thalia, would you care to demonstrate the principles of Rucuun's theorem?"
Ah, Rucuun's theorem; she remembered the first time she'd read about it, at age twelve.  He had been one of the foremost theorists in the field of hydromancy.  It was, not surprisingly a result of her upbringing, one of her better fields.  And it was during the course of her upbringing that she had become acquainted with Rucuun, and then he had in turn acquainted her with his theories.  It had been hours of it actually, Rucuun was rather somewhat of a windbag, especially when it came to hydromancy.  Most boring three hours she'd ever endured, and the last time she'd stuck around for one of her father's parties.
The theorem was that sufficient cold could used to actually stop time.  The principle was simple enough; and she'd long been able to impede the progression of time using hydromancy for years.  As an immortal, she was far more interested in ways of speeding up time then those of slowing it.  But in combat it was a valuable tool.  Even if you weren't fast, and didn't have ninja-esque reflexes, it would give you the time to block swords, deflect arrows, and a sufficiently powerful mage could even walk across a raging battle unscathed.
Of course, it was called a theorem for a reason.  In fact, it had taken her all of ten minutes to completely take apart Rucuun's life's work, which was good because if it had taken any longer she'd have given up on escaping the conversation and just attempted to prove the theorem for him, on him.  No mage could ever actually freeze time, or if they had then they'd never be able to interact with anyone to prove it, since they'd live out their entire lives in a world occupied by statues.  Besides the fact that they'd have to constantly move in order to breathe and everything would be dark.
It hadn't stopped her from learning how to apply the premise.  Indeed, she'd have been hopeless in a fight without it.  Simply put, she just didn't have the frenzied pace that so many mages seemed to have.  Her reflexes were more attuned towards the deflection of falling feathers than to spellfire and sword fights.  Most magic users with her combat skills just took up enchanting or illusions.  She preferred necromancy to actual combat, but she was still very talented with combat magic, if not the rest of it.  And so she'd learned techniques like this one in order to even the odds.

"Yes professor, but I'll need someone to attack me in order to really display its effects."
"Very well, Anden, could you attack Thalia?  I'm told she gave you a good reason to do so on Saturday."
Anden's girlfriend had broken up with him after the match.  Thalia had no qualms about having revealed that he'd been cheating on her; deep down she'd already known anyways.
Anden stepped out of the line and in front of the class.  Thalia paused for a moment, watching him take his stance and ready his sword.
"What, still afraid you can't take me in a straight fight?"
"Please," she said as she stood.  "I don't do straight fights."  She stretched a bit and then tapped the ground, causing a stone stave to grow, taking it in hand and turning to face her opponent.  "I hear you're short a date for the dance; if you can touch me maybe I'll make it up to you."
"Fat chance, skinny !$@#*."  The word censored itself as Thalia wiggled her nose.  Anden began to charge as it did.  She inhaled softly, then began to walk around him.  He wasn't even aware of the change, in fact, only she was.  The whole room cooled, and Thalia seemed to be a blur to all those watching.  She wasn't slowing it so much that he couldn't react at all, even though she could have; it was good enough that he couldn't touch her, but slow enough that he could watch it coming.  She swung the staff at his right leg as she walked by, breaking it over his knee, and dislocating it in the process.  She hopped around him and then jabbed at his left knee, doing the same to it as she had to the right.  Lastly she took the remnants of the staff and walked back to his front.  She took it in both hands and smashed it over his head.  Then at last she exhaled, and watched him slump to the ground.  He wasn't dead, but the point would be solidly made for everyone watching; she wasn't one to be messed with.
Oh right, and that the theorem was very useful.