It kept up for half an hour. Celestia would wiggle down, taunt you with a rear end pun, bounce up and down on you--all to make you struggle and tickle her flank. Sometimes she's sit still for minutes at a time, simply enjoying the feeling of your tiny form pinned beneath her.
Then, she decided to try something new.
***
Celestia leaned forward, panting and fanning herself. When she'd put more weight down, the human's desperate struggles beneath her rump felt...amazing. The feeling of totally dominating the small being, how each thrash was like a massage for her rear, the muffled yells...it was wonderful.
And of course there was the thought of using her BEHIND to play with the human. It was so childish and even a little perverse, and that made it even more fun! And her flanks were the perfect size for it. She leaned back again and added a few twists of her flanks for good measure, grinding the human beneath her even more. Their struggles grew more frantic than ever, and her face flushed. She laughed. "Oh, a fighter are we?" She taunted, looking down. "Fine. This is our counterpunch!" She slowly leaned back on the cushion, pressing down more and more, more than she ever had until now. Soon, her full weight bore down on the human---
***
The pressure skyrocketed. Agony shot through your body as Celestia's flank bore down harder than ever--this time down to your bones. And the weight was only increasing!
You couldn't move, couldn't scream; everything hurt too much. It was horrible, it was a nightmare, it--
***
*crunch*
Celestia's eyes shot open. She suddenly felt cold, her euphoria gone. The human had suddenly stopped that lovely struggling, and she couldn't really feel their body's imprint on her rump anymore--it was as if the human was suddenly flat.
Oh dear. Did I really...? She stood up and looked at her rear to confirm what she already feared. There was nothing left of the human but a small red stain on her backside--the creature must have popped like a grape once she put her full weight on it.
Celestia felt slightly guilty. She knew full well how dangerous sitting on the human was, and had given little thought to holding back. But she hadn't intended to simply crush the human like that; it was a terribly undignified way to die.
She couldn't help but smile as she took out a piece of cloth and wiped the human's remains off her left cheek.
Oh well. It was fun while it lasted. I may as well tell Luna what happened...she'll be upset about me crushing the human, but once I tell her how--exhilarating--sitting on them is, she'll cheer right up.
Celestia headed for Luna's chambers, humming a happy tune. At some point in her her walk down the hall, she dropped the human-stained cloth down a laundry chute--anypony who saw it would never know why it was bloodied, simply assuming somepony had a nosebleed.
Celestia, for her part, never gave the human she'd crushed like a bug under her sitting rear a second thought.