Jerk him off

"Do you like that, master?" you ask, working his cockhead in your fist until he squirms with pleasure, his body shaking as he gasps and shudders. "Do you like having a guy play with your cock?"

"Y- yes!" he cries. "Yes! Please, don't stop!"

You grin and wrap your other hand around the base of his shaft, stroking the length of him while you keep polishing his knob. Convenient that all demons are ambidextrous. You lean back down and press your face into his ass, running your tongue around his hole a few times before moving down, licking and sucking the flesh between his balls and anus. The skin beneath your mouth tightens, his balls drawing up. You capture one of those globes in your mouth, sucking as you roll it with your tongue.

"Oh! Oh, fuck, I'm cumming!" he shouts, bucking into your hands as his nuts contract, his cock pulsing, warm strings of jizz filling your palm and dripping between your fingers. A heady rush of sexual energy floods through you, filling your body. You feel stronger, your body slightly larger. Even your cock looks a little bigger. You keep stroking him, harder and faster, milking every last shudder of pleasure from his sweaty, shaking body. Finally, he collapses on the cloak, spent and exhausted, his cock softening and slipping from your grasp. You sit back and lick the cum from your fingers as you regard the dazed and gasping human before you.

Such fragile things, so easily worn out. It'll be an hour before he's 'up' to fucking again. Well, maybe a half hour. He's young. But still, it means waiting, and you hate waiting. You feel the power surging through you, and you want more. With this power, you can change your body to make you more effective at pleasing your partner. Bigger cock, longer tongue, a more dexterous tail...your predecessors have tried them all, and you smile at their memories, the successes and the failures. Then a memory really catches your interest, about chemically altering your body to produce pheromones that increase the human libido, making them horny and virtually insatiable, capable of fucking for hours and cumming repeatedly. Sounds like exactly what you need.

But to do that, you'll need more energy.