A large mirror, in the mansion of a town noble. Auspiciously placed to the side of the foyer, unlikely to be touched without cause (nevermind smashed), and in an area where using it to see the land outside was quite feasible. Some mirrors were blessed sacred artifacts, whose viewpoint of beautification greatened those glories reflected in it and repelled evil. As a demonic cultivator, you had also seen evil mirrors whose cursed reflections ate at the images shown within them, siphoning strength, qi, and sometimes other attributes for its own nefarious ends. Certain formation array talismans made use of this principle to emulate, redirect, and so on. This mirror was neither, but closer to the former. It was blessed with a simple protective charm woven into the silver backing, mostly to keep it clean and prevent certain forms of demons from misusing it.
Demonic cultivator with mirror techniques and sacred fire or not, you did not count as those forms of demon. But you were certainly going to misuse it! Seating yourself on any one young man long-term would mean having to either synergize with or regularly overcome their soul: manageable but inconvenient. Intermediary equipment artifacts could mean quite potent control, but also potential ease of removal. And while you weren't opposed to such rehosting dalliance in a strapping young man, you could transfer later. You needed a power base before you came back into the public eye of the world of cultivation. The crude but ageless immortality of manifest spirit meant that the time to do such was not really an issue. You planned to use the mirror-space pseudo-world to affect the reflections of people around here: through your techniques, this would transform the beings those reflections came from. You would probably go subtly at first in terms of externally visible effects...but possessing the reflections of attractive men and practicing homoerotic dual cultivation with them? Priceless.
Your soul settles on the mirror peacefully. It's young enough it hasn't really solidified an animist spirit, and using it as a technique focus means that if any happens it'll probably not oppose you. You manifest an echo of your old body to explore the world of reflections: a naked not-that-old maturely sexy man, albeit a bit translucent and golden-tinted. A simple illusory technique just inside the mirror's reflections to hide your activities to an outside observer, and you were free to get perverted. By controlling the extent to which your activities in the mirror world affected the reflections outside, you had significant freedom in deciding what was playful and what would actually take effect. Performing some...periodic resets of the reflection as needed for symbolic accuracy was also easy, as was controlling the extent reflections reacted to their changes. How much the people being reflected cared would take a bit more sympathetic technique, but nothing beyond your ability.
The reflected world extended significantly beyond the actual reflections it had seen. For someone of your abilities, it was easy enough for the entire town to be reliably affected just from this one center of spiritual power. You configured the reflected space so that for now the reflections would ignore what you were doing to them and pressure of change compliance was something you'd be able to enforce at will rather than automatic.
And immediately skedaddled through the rest of the house to get your hands on (the reflection of) the mere mortals and lower-stage cultivators. Most sect elders were Core Formation, and most random sect adherents or independent talents were a stage below that in Foundation Establishment. Plenty of dilettantes, trainees, and whatnot were further below in Qi Condensation, typically tied to their specific martial techniques and energy reserves rather than generalist abilities. Several mundane administrators bothered to cultivate primarily for their mental abilities or the benefits of regular exercise. It was one of these you felt like messing with first.
Lord Lingzhu, a born administrator, was a man in his 50s (aging gracefully or age-ambiguously), warm-toned with a short black goatee-and-mustache beard and relatively short hair styled into something mildly spiky. He was slightly chubby-bodied but not really fat, and had enough muscle to keep his gut toned. His reflection currently sat in his office, signing off mirrored papers. He was wearing daopao-style black robes that you shucked off him mostly for the heck of it. Expanding your spiritual sense to bypass matter and pulling him away from the table (causing him to go into a listless autopilot and more or less stand there for perusal), you saw a fat if somewhat-short soft cock and particularly large balls. A few cursory tugs confirmed it wasn't much of a grower in length, but it still got a bit girthier when erect.
Using the sympathetic powers of your technique, you gave the actual Lord Lingzhu a boner. Through simply ordering his reflection, you conveyed for him to not object to random erections or increases in libido. With little nudges like this to test the waters, you'd soon convince men to fuck each other in the streets if that was the cultivation progress rate you needed. Maybe not rush it, to give some time for the Heavenly Sword Sect to assume you were staying dead. A quick resetting of the reflection confirmed that the real administrator had simply ignored the tenting of his robes.
You again tossed those robes aside, but this time stepped into the body and possessed the reflection. You immediately felt up his pecs, playing with this transient body's nipples and confirming the experience of a sensitive chode of a dick. But you had a lifetime of secret demonic fetishes and mastery of reshaping technique, and there were some old favorites you'd be up for trying with a drastically different body. You probably wouldn't map this onto the real Lord Lingzhu (at least, until you'd thoroughly brainwashed everyone who might notice), but...
Tracing your perineum, genitals mystically disconnected as if dismounting an attached armament. This left behind a bare if sensitive surface and what was effectively a dildo that you could feel remotely. Holding manhood in hand, you gave it a few appreciative licks to judge the taste (average, okay-smelling, precum very quickly) and flexed it a few times (quite strong dick muscles, apparently). A little finger-poking on the underside canvas of your crotch, and a vulva formed to offer sufficient depths to store it in. You gently pushed the dick into yourself, and breathed deeply as the new temporary orifice was lubricated enough to handle it but needed to go slowly and adapt. A form of fucking oneself that effectively doubled the pleasure of masturbating, and by returning semen to the body easily recaptured any qi coincidentally embedded. A fantasy technique for sure: easier in spiritual spaces like reflections, but still plenty possible in local reality. Fun as it was, your purpose here was to check the spiritual energy averages in informal edging and orgasm. You didn't bother to perform any specialized technique, more or less slowly easing yourself back and forth, up and down into self-fucking and tracking the general vibe. Lord Lingzhu was okay, but not particularly impressive. Given that most talented people went for the sects or adventuring, seemed about right an administrator getting it on wasn't particularly earth-shaking. After a while you sped up and stopped trying to hold back, firing string after string of qi-charged cum into the spiritual cauldron of an artificial womb. It was...pretty alright as far as self-touching went, but definitely going to need significantly more sluttiness to keep par with other cultivation techniques.
You sighed and reset the reflection, a 'poof' transformation to conventionally male conventionally dressed Lingzhu at his desk and you ejected from his body. You did appreciate that he was apparently touching himself inappropriately under his desk, so mission accomplished at kickstarting corruption or whatever. It was a nice brief excursion to get started.
Looking through reflections of random documents in the office, you assessed possible targets. Lord Lingzhu's wife was long gone, apparently a Foundation Establishment cultivator who had died in sect-to-sect violence for defense of the state, leaving behind a young adult son who lazed around the house, Guanzizai, long-haired pretty-boy type. There were a few Foundation Establishment guards and a butler, but everyone else was middling Qi Condensation at best including Lingzhu. Since it would take multiple Core Formation experts (or perhaps one with a really nasty surprise attack, or type advantage countermeasures of similar power) to stop your ministrations, you could pretty much have your way with everyone here. Simultaneously and openly even, if not for wanting to keep attention on the down-low for a while. Lingzhu answered very nominally to the Alchemical Tower Sect for protection purposes and where talent was sent, but the town of Twiceriver was more or less freely administrated. Because the Alchemical Tower laid down very little actual law and most of its higher-stage cultivators were in the titular tower doing alchemy, go figure. All around, ripe for the pickings.