Friendship has benefits.

Isaac probably could have just asked to teleport straight to Twilight's crystal castle, but walking through town had turned out to be a lot more fun, getting him nice and riled up. His cock was firmed up down one of his baggy pants legs, making his walking gait a bit more stiff, but it was such a novel sensation that Isaac didn't mind one bit. He really was the big man on campus!

The lower levels of the castle were open to the public, doubling as Ponyville's library, so when Isaac reached the big double doors, he let himself into the foyer. "Hello?" he asked, his voice echoing slightly down the empty crystal halls. Not a whole lot of ponies around at this time of day. "Twilight?"

From somewhere else, Twilight Sparkle's voice echoed back. "O-Oh! Isaac, is that you? J-Just gimme a second! Ngh!" After waiting most of a minute, Twilight emerged from one of the library chambers, dressed in a blouse and skirt that it looked like she had just thrown on. Her mane had a few hairs askew, there was still a tinge of red on her purple cheeks, and her legs were held stiffly together. Frankly, there was no mistaking what she'd been up to. "Y-You're back! From Canterlot! How did your meeting with Celestia go? Did she help you with your culture shock?" she babbled, barraging him with questions to distract from her own state.

Isaac chuckled and rubbed the back of his head bashfully. "I get the feeling I... interrupted something."

Twilight waved both her hands up by her head. "No no, it's okay! I've always got time to help you, Isaac! You're kind of my responsibility, after all, since I found you! Haha!"

As cute as it was to see Twilight stumble over herself, watching her average-for-this-world curves wiggle beneath her hastily thrown-on clothes, Isaac felt a need to speed things along. "You can just admit that you were masturbating, right?"

Twilight let out a sigh of defeat. "I know it's obvious, but you're a male from a more modest world, so I didn't want to-" She stopped abruptly and looked at him with wide eyes. "You... You're... okay with that? You're not... embarrassed anymore?"

This confused Isaac for a moment, before he remembered that he was in a retroactively altered timeline. Before he found the Root, his problems stemmed from his masculine pheromones making everyone standoffish towards him, on top of being an awkward alien stranger whose novelty had worn off. But in this timeline, his social problems were now largely agreed to be caused by this world being so casually sexual, causing culture shock and friction as both parties didn't know how to act around each other, especially since Isaac's masculine pheromones were naturally super-attractive to everypony.

Still, nothing that a day of fucking both high Princesses of the land couldn't fix!

Isaac chuckled and put his hands on his hips as he figured out how to 'play along.' "Yep, Celestia kinda helped me get my head on straight about everything. Gave me a lot of context for why things are so sexualized in this world, too."

Twilight pouted and wrung her hands beneath her bust. "I'm so sorry I wasn't up to the task. This whole situation was unlike anything I'd ever imagined. I mean, a world where sex is taboo?"

"Nah, I totally get it," Isaac said reassuringly.

An awkward pause fell between them. Both of them were natural nerds, after all. Awkward small talk was not a strong suit for either one of them.

It was Twilight who broke the momentary ice. "Sooo... uh... if you don't mind me asking... how do you feel about our world now?" she asked with a nervous chuckle. "I mean, you're a male unlike anything we've ever had before, so we're all very curious about what you're interested in...!"

Isaac couldn't help but smirk a little. It was obvious that there was a hope, however small, that she was looking at a potential Harem Master - a true male that would fuck and claim her. The tension in the air was delicious, especially with him knowing in advance where it would inevitably lead.

"Look, Twilight, I've gotta be honest..." Isaac began, keeping his tone ambiguous as he took a step forward.

Twilight nodded and visibly braced herself for the worst-case scenario. Not that it was gonna happen.

"I wanted to fuck you all from the very start."

Twilight's big purple eyes went super wide. She audibly gasped.

Isaac grinned, then continued a bit more earnestly. "I just didn't want to offend any of you and lose my shot. It's one thing to be aware that you're in a world that treats sex more casually, but it's another to really get the subtle nuances. The social cues and signals. What the unspoken permissions and lines are. I wanted to approach you, your friends, just about everypony in town at one point or another... but whenever I thought about it, it felt like staring down a social minefield, because I didn't know if I'd be welcomed in your sexual culture so easily."

Twilight listened to every word with rapt attention, her breath shallow as the tension in her body built. "I'm so sorry it was like that for you. But... Celestia helped you with that?" she asked hopefully, taking a step closer to him, bringing her breasts closer to bumping into his chest.

"She did. She knew what it looked like from the outside and helped me understand what I was missing from the inside. The issue with your males, the effects of my pheromones, the whole Harem Master concept..."

Twilight let out another small gasp at that. Her thin blouse dented from her nipples firming up at the mere mention of the idea. Her face grew more red, her breath came in heavier - the sexual tension was almost palpable in the air. "I-It's not really an 'issue' with our males so much as... we're all kind of on the same team... S-Sorry if I left you in the dark - the whole Master thing, it- it's a lot of pressure to put on one guy, and we were already giving you bad news about ever going home, so then to try and press-gang you into taking on such a monumental responsibility - it seemed so very wrong."

Isaac nodded and took another step forward, bringing his body in contact with hers as her nipples brushed against his chest. "I appreciate that, Twilight. I think you were right overall. I just needed time, and maybe more direct help. But now..."

"Now...?" Twilight's big eyes looked hopefully into his.

It struck him that they were about the same height, which seemed odd after fucking two mini-giantess alicorns. "Ah, sorry, I got distracted. You're really growing into your alicorn physique and curves, aren't you?"

Instantly, Isaac was looking up at Twilight rather than straight at her. She stood a full foot higher than him, broaching seven feet tall. His hands were on her arms rather than her shoulders. And best of all, her enlarged tits were now pressing against his face, framing his cheeks in her cleavage. Twilight smiled sheepishly down at him. "I'm nowhere near Celestia or Luna yet, but... Please, Isaac, what were you about to say? Do you really want to fuck all of us? Dominantly, not just out of courtesy like our own males? Are you saying...?"

Isaac nodded, letting his hands drift down Twilight's arms and down to her hips, confirming for himself that Twilight had retained her balanced hourglass figure - she had just grown out all over. "Twilight... I'll stop beating around the bush. I want to become the Harem Master."

Twilight gasped yet again and stiffened in shock. He could feel her body warm up as she pressed against him. He could almost hear a faint squelch as her pussy clenched.

Grinning in delight at her reaction, Isaac chose to keep piling on. "I fucked Celestia and Luna last night. And now I'm on a mission: To fuck you and all your friends, plus anyone who catches my eye while I'm down here. And then in about a week and a half, I'm gonna return to Canterlot and fuck my way through a summit of all the world's leaders. If all goes well, with you and everyone's support... I'm gonna declare myself Harem Master of the World and claim everyone on this planet as my slaves!"

"Kyaaaaaahhh!!!" Twilight screamed up to the crystal ceiling. Her legs gave out as her pussy splashed juices onto both her and Isaac's legs. She tumbled forward, and Isaac had to back up and steady her by her tits so that she merely fell to her knees rather than completely on top of him. Isaac then took a step back and admired her blushing, panting, shivering form, her nipples trying to poke through her blouse, her skirt soaked in the front, all attempts at modesty forgotten.

Isaac's massive, oversized cock was now straining against his pant leg, begging to be freed.

Twilight reopened her eyes and stared up at him, not too much lower than his eye level despite kneeling thanks to the size difference. Her chest heaved and her legs squirmed. "Y-You're serious, right? You're not... This isn't some prank to try and tease me...? Please...?"

Isaac smiled broadly, smugly. "You could always pull down my pants and find out."

The Princess of Friendship lunged at his crotch.