Experienced knight defeats EVIL WIZARD in the mage's stronghold, wizard uses non-magical charm and suggestive words to convince the knight (as he's being dragged out) that the conquest wouldn't be complete without degrading the mage by forcing him to suck off the knight

Knight becomes convinced, does it in the middle of a large room/chamber between the mage's "throne room" and the entryway, mage gives great blowjob but whoopslol, turns out he used the energy released to activate a spell circle hidden in the engraved floor of the room, turning the tables when the knight is off balanced.

The knight becomes magically bound as the mage uses magic on the knight, causing a second orgasm - much stronger than the first, with a lot of jizz grabbed in a magic aura and floating. Knight feels weak after that, his armor feels off. Mage disintegrates the armor and clothing, leaving the knight nude - and now looking like he's 20 years younger, about 20 or so, and in the process lost a lot of muscle mass - and he can remember learning stuff but it feels only like a memory, his training is gone as well (XP drain).

The mage gives a speech, knight says "you'll never win", mage says "I already have", and blasts the knight with another magic spell, sinking in as the knight is hit wtih antoher orgasmic XP drain, the aura holding his last ejaculate and ejaculated XP grabbing this one, the knight now drained of his Strength and Stamina (DnD terms, he was 20/20 at start, 16/16 in the first stage, and now is 10/10)

Knight pants, "Are you done yet?" Mage says "No, one more thing to take," then strikes the knight in the stomach with another spell, the knight's frame shifting and changing as muscle done decreases further, becoming less and less masculine and more androgynous, those stats also being collected by the floating aura of magic, now a fairly large blob.

 "And of course,  one of the guidelines of magic is equivalent exchange; when you take something you have to give in return, yes?" Another orgasmic spell hits the knight, his shoulders pulling in further, his middle and waist feeling like they're in a vice as they compress, while his hips ache, then swell larger, his hands and feet becoming more dainty. His flat chest tingles then grows into breasts, first A-cups, then B-cups, the nipples becoming firm and hard. 

His facial features soften further, the remnants of his facial hair receding into his chin as his Adam's apple shrinks and vanishes, leaving his voice higher pitched. His scalp tingles as his hair grows out from his close crop down to his neck, then shoulders, then to the middle of his back. 

His much-shrunk balls pull up within his widened pelvic cavity as his cock shrinks to become a small nub, a rolling wave of ecstasy building in his middle as he completes the gender transformation, as her womb fully forms, the pleasure cresting in waves over her as she experiences her first female orgasm, leaving her dripping with fluid going down her thigh.

"What..." the knight pauses at the sound of her new voice, then continues, suppressing a small quaver, "what have you done to me," "Oh, just the start. You see, when you rampaged through my castle, you managed to kill a good portion of my gnoll army. In fact, I don't have any males left! I'm afraid that this just wont do." The knight pauses, the putting on a brave face, "Oh, I'm so sorry, is there anything I can do to help with that?" she asks in a mocking tone of voice, then goes white as the mage grins, "Why yes, there is! Thank you for offering to help," before blasting her with another spell, the sensation of transformation coming over her - but slowly, as her frame swelled a bit here and there, her hips widening even further, breasts swelling a big to become large Bs, verging on Cs. 

Her groin tingles as her slit changes and shifts, becoming something not quite human as a tingling sensation in her new womb appears, only a slight sensation compared to the rest. Her pubic and armpit hair thicken, becoming patches of fur, slowly making their way across her form as it continues it's slow warp. 

The wizard grins, watching the knight's torso shift, the fur growing spots in places before meeting the knight's now plump ass. The wizard outright laughed when the knight's new tail suddenly burst from her spine and left her gasping, "I could have had this transformation happen faster,  but that would be no fun at all. Instead I'll make you feel every bit of change as you lose your humanity, and watch the next step."

"Now, I have an apology to make. You didn't kill all my male gnolls - I have one young undersized runt." At that, a small and remarkably well dressed gnoll, holding some scrolls, shuffles forward to where the knight can see. "You wanted to see me, Master?" the gnoll said with remarkable diction." "Ah, there you are. Now," he turns to address the knight, wincing as her legs shifted, thighs bulking up while her calves shortened, her feet beginning to stretch longer, "He doesn't look like much, but his grandfather was  the strongest gnoll to ever live. And with your generous donation, we will unlock his potential and shave a few years off training him."

At that, a look of fear passed over the gnolls face, "No, Master, I don't want th-" the gnoll let out a loud yipe of mixed surprise and fear as the mage's aura grabbed him, restraining him in the air much like the knight. "Fool. It matters not what you want, only what I need."

While the gnoll futility struggled against his master's magic, the knight's transformation continued, her ankles, knees, and hips shifting, giving her a natural digitigrade stance, while her feet lengthened, then the balls and goes surged wider and thicker, throwing thick pads and dark blunt claws as they became canine in nature. Fully furred from the waist down and having tendrils of fur wrap around her stomach and chest, three pairs of blazing warmth suddenly sprouting before fading, revealing 3 more sets of nibbles in rows below her first set of breasts, then the flesh below slowly begins to swell and grow.

With a wave of his hand, the knight's former masculinity, virility, and experience slams into the gnoll, covering his body. Tendrils of fluid snake out, seeing entry into the young gnoll's body before one snakes down into the trousers and into his virgin tailhole. The sudden pressure of fluid and stimulation of his prostate made him gasp, and a second rope of fluid entered his mouth, forcing its way down his throat. After only a few seconds, all of the fluid had disappeared inside the gnoll's body, and the mage released the magical bounds on his servant, who fell to the floor, on his knees and hands, moaning softly from the abuse before suddenly stiffening, and letting out an anguished cry of "No, no!"

TLDR for next bits - gnoll grows older and larger, undergoes brutification and IQ drain, bursts out of his clothing. Knight's transformation continues, with a growing arousal and heat as the male gnoll matures/grows, her last human features of face, nose, and muzzle shifting as he howls, the beast having displaced  the old personality entirely. The mage releases her bonds but she can't flee, her mind fighting submission as the alpha gnoll approaches, then makes it moot by grabbing her, licking her snatch a few times, then breeding her, the mage using magic to enhance things. 

She's already showing signs of pregnancy before the alpha even pulls out, her body becoming softer and more rounded, her breasts becoming larger, heavy, then dripping with milk, and a rapid pregnancy, coming to term with a litter of eight large pups, then giving birth. The pups suckle and quickly drain the knight's breasts, and are growing rapidly and are already past the toddler stage before the mage's other servants take them away. 

The former knight is exhausted but then feels a stamina restoring spell on her, then sees the alpha gnoll again, already responding to her magically restored heat, "And with a litters of that size, why, it'll only take... hmm, about a month before my army is restored." The mage puts a pendent on the former knight, weird enchantments coursing through her body, "And with these fertility, stamina, and sustenance enchantments, you can go nonstop! What fun!"