Watching the sunset with a fertility goddess.

Isaac and Celestia retired to the latter's private quarters, which were even more of a love-nest than the top of the throne room had been. She practically had a whole suite capable of hosting many guests at once on a giant four-poster fuckpile bed. Right now, though, it was just the alicorn and her big-dicked human, with him laying beneath her breast while she stroked his erection.

With his face muffled by an enormous pure-white breast, Isaac murmured, "Is breastmilk in this world magical, delicious, nutritious, and can be drank endlessly without filling up or bloating the one who drinks it?"

"Hmm? Did you have another question?" Celestia asked, using her horn to magically lift up her breast to give Isaac a bit more breathing room.

Isaac chuckled and shook his head. "Not yet, just thinking aloud. It's been a heck of a day, after all."

Celestia nodded with an understanding smile and let her breast roll back onto Isaac's face, letting him play with the giant boob to his heart's content while she continued to play with his upright cock. She let out a happy sigh as she did so, relishing the feeling of a hard cock in her hands.

Isaac wasted no time suckling on one giant pink nipple, barely able to fit the enormous protrusion into his mouth. His hands reached up and squeezed and wobbled her right tit, doing everything he could to stimulate it and get it churning. His efforts were rewarded after a few arm-tiring minutes when he at last tasted something warm and sweet on his tongue, preceded by a shuddering gasp from Celestia. She then used her magic to pull him in tighter into her lap despite how much real estate her breasts took up, smooshing their bodies closer together. Her belly was at last mostly receded from all the cum she had swallowed and absorbed, so there was enough room on her plush thighs for him to lay upon. Isaac, for his part, just closed his eyes and lost himself in the sensations, gulping down mouthful after mouthful of enviably potent alicorn breastmilk. Each swallow seemed to energize him even more, making his cock throb and strain a little further.

"Ah, so humans are receptive to the powers of milk~," Celestia observed with a coo of desire. "Hmhm, as if you needed help outlasting a roomful of ponies and going for more~. But I, or any mare, will happily help you 'recharge' whenever you need it, Master."

Once he'd had his fill for the moment - yet somehow, in defiance of all causality, not feeling very full despite the lingering aftertaste - Isaac lifted the huge nipple out of his mouth and let it leak onto his bare chest for a bit, idly rubbing it with his hand. "Hey, I'm just a boob-hound at heart who loves to see them wobble and spray. If they have a practical application, that's even better."

"It did not escape my notice that you went for my breasts first thing," Celestia teased him, "though that didn't stop me from feeling vindicated for spending the last thousand years making sure my breasts are as perfect as they can be. I wonder, though - how does that make you feel about all the stallions that will no doubt be pining for you soon, even if they don't have naturally big breasts?"

Isaac shrugged, and his cock twitched at the idea. "I dunno... I'm not opposed in any way. There's something about dominating another being's cock with my own, I'm discovering. I like being superior. I like being the 'only' man, the 'real' man. I like other males, especially males with some real size to them, bending over backwards to submit to me. So, I mean, I'm always gonna drift back to my comfort zone of huge tits, but variety is the spice of life, right?"

Celestia grinned and gave his cock a passionate full-handed squeeze just beneath the crown. "That was a very good answer, Isaac. You are self-aware but have an open mind - very useful traits for a prospective Harem Master to have, indeed."

Now seemed like as good a time to bring up the elephant in the room as any. "Chrysalis mentioned..."

"...that there were other candidates before you, yes, I figured she would," Celestia finished with a sigh, her stroking of his cock slowing down. She stared at his cock and licked her lips like she was reminiscing about cocks in her life before. "If there's one harsh lesson I've learned, it's that being the Harem Master can be just as hard as the male himself as it can seem demanding on the creatures around him. But in truth, all we have to do is obey our instincts and get fucked. It is the Harem Master who needs to muster enough lust to match the needs of an entire world. By comparison, we have it gloriously easy."

Isaac was genuinely surprised. He hadn't expected to hear such a perspective regarding a reckless juvenile power fantasy like this. "But..." Man, he really needed to not phrase this like a question! "Honestly, I would've thought you'd have people busting down the door to try it out. It's basically any greedy male's dream come true, having a free pass to fuck anyone he likes at any time. I mean, obviously there's logistical challenges, but..."

"You'd think that, wouldn't you?" Celestia responded with a melancholic smile. "But we've spent centuries working out the logistical details in advance. All the species of this world will be happy being in a harem no matter what size, even if many of us never even get to fuck the Master directly due to there not being enough hours in the day. Once we are Branded, only the Master's happiness matters for our sexual satisfaction. But no, it turns out the weak link of this whole arrangement, after all that... is the Master's happiness."

The way Celestia looked at him as she said that, her face equal parts deeply empathetic, genuinely caring, and legitimately worried... Isaac wasn't sure how to respond.

"For what it's worth, I think you're the one. Truly," Celestia assured him. "But it will depend on you. Promise me, Isaac... Always be honest with yourself and how you feel. If you lust for something, tell us, no matter how greedy it is. If you need to stop, just want to cuddle, or just want some time with your own thoughts, let us know. If you are struggling with complicated feelings, talk it out with us. If you find yourself craving more purpose in life than just being Equestria's dom, we will support you to the fullest. This isn't about meeting an orgasm quota. This is about living a life of desire to the fullest, with no regrets. That is the path of the Harem Master."

...I mean, I've been marked by the Root of Honesty, so technically I'm always honest with myself - I decide what's honest! a small part of Isaac snarked. But Celestia's words did not fall on deaf ears. Even though her wisdom had been twisted by his reality changes, bent through a lens of serving him eternally and satisfying his endless greed and lust, it still rang true. "I promise, Celestia. All I really want, after falling through a random portal and getting stranded in another world, is to live a comfortable and happy life where I finally put myself first. That's it."

"Ah, you've spent long enough serving others?" Celestia deduced with a playful smirk. "Time for everyone to bow to you?"

"That's the general idea, yeah," Isaac replied with a grin, allowing himself a moment to show his truly greedy side.

Lowering her voice into a raspy, sultry drawl, Celestia husked, "Then tell me what you want, my dear Master. How do you want your ****, your slut, your whore to serve you?"

Isaac grunted as a wave of lust surged through his body and cock. "Gngh... Keep praising me, keep stroking me, and then drink down my cum...!"

"Your wish is my command, Master," Celestia cooed, then leaned down to give his cock a long lick across the tip. "You're the greatest Harem Master candidate I've ever been fortunate enough to witness. Your big human cock fills me with awe and lust. It takes all of my willpower not to drop to my knees and hand you my crown whenever I see you. You're beautiful and brilliant, not just in body but in mind and heart. No male of my own species has ever made my heart race like you do. I will follow your lead to the ends of the earth."

Isaac panted and pulled Celestia's milk-dripping nipple back into his mouth, suckling on her larger-than-life titty once more as he listened to Celestia's honeyed words of praise. Grinning with delight, Celestia sped up her stroking.

"My tits, my ass, my mouth, my pussy, every inch of me from the tip of my horn to the bottom of my hooves, body and soul, magic and wisdom, it all belongs to you. I was put here in this world to welcome you into your sexual destiny, I'm sure of that now. You're going to make my ponies so happy, and make them cum over and over and over again until they can cum no more. Soon just looking at your cock will be enough to enslave them on the spot, to make their pussies quiver and their balls empty. And that's before you touch them, before they get a whiff of your manly scent, before they taste your addictive cock and cum, before they are immersed in all that is you. Because you are not just a lucky mortal. You are on your way to becoming a true... sex god. And we will all soon worship you."

Isaac's legs quivered and his balls tightened. Celestia craned her neck down and latched her lips around the head of Isaac's cock just in time to catch the first eruption of cum, slurping it up into her mouth and down her throat shamelessly. The room was shortly silent save for the sound of Celestia gulping down thick mouthfuls of viscous semen, her eyes closed as she focused on her singular task.

Thankfully, Isaac's body was better prepared to handle such intense orgasms, but they still took a bit of wind out of him. When his body was done tensing up and convulsing, he practically fell as lump as an eel across Celestia's thighs, unable to lift a finger for the moment. He laid there, dumbstruck, milk drooling out of his mouth while he caught his breath. And Celestia made sure there wasn't a drop of cum left over on his cock.

"How was that?" Celestia asked with a naughty giggle. "I'm sure my dirty praise will get better the longer I get to know you, figure out which buttons to push, but just then I felt like that was a bit scattershot."

Isaac just raised a thumbs-up from where he lay. "It did the job."

Celestia laughed with shameless joy. "That warms my heart to hear, Isaac. I will gladly make you cum however you ask for as long as you'll let me." Her horn lit up, and her magic lifted him up off her lap to place him on the giant bed proper. "If you'll excuse me one moment, though, my duty calls. The sun must set and the moon must rise."

"Oh, of course, yeah..." Isaac murmured, too stuck in the afterglow to argue. He just rolled over and watched Celestia wobble out onto the balcony of her private quarters, gazing out into the sunset.

Her giant breasts and substantial booty and thighs cast an incredible shadow against the sun's light, a holy silhouette of pure exaggerated fertility. Her horn glowed with the same golden light as the sun and she rose into the air, incredible power swirling around her as she became in sync with the life-giving sun. On another balcony, the same thing happened with another princess in the silvery light of the moon. The skies slowly darkened as the sun retreated and the moon rose above the land, revealing the stars in all their glory with it.

It struck Isaac that there was nothing particularly sexy about this whole magical phenomenon. Well, it was sexy that he was fucking exotic females that could command such power, but it wasn't really the point. Even after all of his perverted changes to reality, fundamentally this was still a fantasy world of magic, where amazing things happened like magical winged unicorns moving the heavenly bodies.

Then the shadow of an absolute dumptruck of an ass passed over the balcony, and Isaac knew that his next trial was approaching.