Perhaps he should have checked what her dream were before promising to fulfil them

Rarity froze. She dropped the sapphire and it rolled across the floor. Immediately, Spike felt the atmosphere change from delight to dread and his insides churned in fear. He wracked his brain for a reason behind what seemed like an **** overreaction, trying to find where he’d gone wrong. Rarity loved to talk about her dreams. Her career, her Prince Charming, the new dress she had, and most annoyingly to Spike, whichever crush she happened to have. She’d never been embarrassed before, so why was she now?

"You have to keep this a secret between us, Spikey. Not even our friends or dear Sweetie Belle can know this. I think I'd just die from embarrassment,” Rarity explained. Sitting next to him, her voice shrill and wretched with fear. Her fingers were fidgeting and her white fur was tinged scarlet.

Panic spread through Spike’s blood. He hadn’t meant to embarrass Rarity, and he was quick to try to think of a question that would save her the embarrassment.

He had one. 'You can avoid talking about it if it embarrasses you, can't you?' He could use that.

"You wouldn't be embarrassed talking about this. It's completely fine for you to talk to me, isn't it?" Spike said instead, then immediately covered his mouth. What was that? He wasn't trying to say that. His lips worked on their own.

The spell however didn't care if he hadn't meant to say it though, the magic still applied and Rarity immediately relaxed and let out a small laugh. "Well I suppose that's true Spike, you're the only one I feel relaxed talking about this with."

She ruffled his head as she pondered how to respond. Despite his panic, Spike listened in closely. He couldn't help but be curious. "My dream Spike, my greatest dream is just a fantasy you see, and while there's a part of me that wants it to come true, I know it won't."

"Why wouldn't it?"

"It's... Well, I admit it's quite the filthy dream, and alas, not all dreams are meant to come true." She admitted to herself with a twinge of disappointment.

"Filthy?" Spike gasped, that was far and away the last word he'd ever expected to describe Rarity's dream.

"You see darling, in my dream, I see a man, a big strong man sitting on a throne. Strong, handsome, exotic and powerful, but he's not alone. He has a harem of willing, beautiful, sexy mares utterly infatuated with the man, you see. They are his property and sex slaves." She explained, her breathing slowly starting to increase. "He's the most powerful man in Equestria and the world, with the ability to add any mare to his harem and any stallion bow to his whim."

"I... I..." Spike gasped as Rarity's face turned a soft pink.

She didn't relent, however. "I admit Spike, I'm downright vile when it comes to this fantasy, because you see, those mares aren't just any old mares. They're our friends, the princesses, all my celebrity friends, and even my little sister and her friends, all devoted to this man's pleasure."

Spike was stunned beyond words now, his eyes frozen on her face, watching her breathing become rapid, and drool slip from her mouth. "And then there's me. Out of all the women in this man's harem, I would be his queen, the diamond in his horde. I would be his favorite of all his consorts, the queen of all others, the best fuck out of all of them. Better than my sister, better than my friends, better than all the celebrities in Equestria, better than the Princesses themselves. Oh!"

By this point, Rarity was drooling, her nipples could even be seen through her bra and shirt, her face a full scarlet and her eyes had rolled back into her skull before it seemed that the effects of the questions have expired and she came back to reality. She wasn't embarrassed, but she no longer felt the need to discuss her dream.

"Excuse me Spike, I'm feeling a bit exhausted. Would you please give me some privacy?" She asked, almost shyly. "Thank you for the gems."

"Um, sure." Spike gasped, taking the first moment he had to run out of the room, having suffered far too much information. Now though, he could run back to the castle and-

'Make Rarity's dream come true.'

What? No, he didn't want to do that. That was just a dream, a fantasy. But then he remembered what he said... And the question he'd asked afterwards.

'You know I'd do anything to make your greatest dreams come true. Isn't that right?'

"Oh shit."

A/N: Writing this for fun, but suggestions, questions, ideas, feedback or your own chapter are always welcome. We'll see how far this idea goes. Leave comments on how exactly Spike is going to have to fulfill Rarity's dream, and who would be the first he's going to introduce to it. (Rarity will be left till later.)