oh, your alien race propagates by converting entire planets? well i recommend starting with a kobold
Niv let out a sigh as they leaned back on the park bench, relaxing and taking in their surroundings. They crossed their arms behind their head and took a look around. The path in front of them was vacant, and only in the distance could they see a few other people wandering around, mostly joggers at this time of day. The kobold enjoyed the tranquility that the early morning offered, and often visited the park when they needed someplace to relax.
Today, though, fate had other things in mind. Niv looked up at the sky, and saw something bright and shimmering. They would've assumed it was a plane, if not for the way it kept growing bigger. In the span of a second, it shot out of the sky and crashed into a patch of dirt barely metres from the bench. Niv ducked to avoid the shower of mud and grass that the impact had thrown, before getting off the bench and taking a closer look.
In the dead centre of the impact crater was a strange metallic object, or device. It was mechanical in nature, and possessed a concave angular shape, curving around like a shallow 'V'. Despite having landed in mud it was still entirely clean, its metal surface polished and gleaming.
Niv leaned down and wrapped their claws around the object, but almost fell forward when they tried to pick it up. It was a lot heavier than they expected, but using both hands and their thick tail for counterbalance, they were able to lift it into the air.
They only had a moment to look it over in their hands before a powerful shock zapped through their arm. In an instant, they lost all control of it, and it began to lift the device behind their head and plant it firmly onto their neck.
"W-what the hell?!" They could once again control their arm, but the damage was done. The moment the device touched the back of their neck, it wrapped a series of thick metallic plates around the kobold's neck which firmly held it in place. The heavy metallic neck-brace caused Niv to lose their balance, and fall back onto their tail. They desperately attempted to claw the device or its plating off of their neck, but their claws didn't even leave a scratch on its surface. The device's progress unimpeded, it continued its task. Niv yelped in pain as it drilled into their spinal column, but moments after it began, the pain ceased. What's more, they no longer had any control over their body.
Everything below their neck went limp, but what they lost in control, they gained in the form of voices filling their head. They spoke in languages they couldn't hope to understand, but along with it came an ever-increasing white fuzz. The fuzz only grew louder and louder, consuming their very consciousness, their identity, their memories, and then, nothing.
Niv was still paralysed, but something felt different. The very texture of their thoughts had changed, and somehow, they intrinsically understood that their mind and brain had been digitized.
"Hello? Helloooo? Can you hear me yet?"
The voice was now clear as day, but nothing about it had changed: The kobold was now able to understand the alien language perfectly. They thought about it, and as they thought, they realised that they were thinking in that language too. They paused for a moment, and tried to come up with any statement in English, only to realise that they couldn't. Niv's language, right down to their thoughts, had been replaced.
"I... I can hear you. What's happened to me?" They were speaking, but not out loud, again, it came perfectly naturally to them, and it was something they never could've imagined before.
"Hi! Oh, jeez, well, that's a long story, but I can give you the basics!"
"Okay, well uh, what's the thing around my neck?"
"Oh, that's simple! It's a biological conversion device! As of now it's only replaced your nervous system, but if you give us permission, we can begin the roboticisation, and make you one of us!"
"Wait... Why? Why do you want to make me a robot?"
"Well, we are a race of conscious mechanical beings, and it's really hard to just create new life out of nothing, so the only way we can really reproduce is by converting sapient organic creatures like yourself!"
"I understand... Yes, that makes sense. But, why me?"
"Oh, we just did a quick scan of your planet and chose which of its citizens were determined to be most amenable to the concept of no longer being flesh and blood! After interfacing with your networks, it sure seems like you enjoy the idea, no matter how secret you try to keep it!"
Niv would've blushed if they still had any form of connection or control over their body, "I-I won't deny that this is pretty, um, appealing! So, uh, what happens next? And what would happen if I said no?"
"I'm not sure! I'd have to ask my owner, because nobody's ever said no to one of my recruitment pitches! After planetary conversion is underway, we usually stop requiring permission, though. It's a shame, really, the unwilling pretty much have to have a total mental rewrite! But you get to keep all your memories and personality, so that's nice! Now, simply say that you give your permission to have your organicness stripped from you, and we can begin!"
It didn't take long for the kobold to make a decision. "I give my permission!"
The device on Niv's neck whirred back into action, panels folding outwards like origami and extending down their spine. The angular plating they were gaining quickly took on the colouration of the scales beneath, turning orange as they spread down the kobold's tail. Each and every panel extending over their body embedded itself into the flesh below in preparations of replacing it, a process made painless by their now-digital brain. Robotic panelling now extended down the majority of their body, encircling their limbs and tail. Looking down at their body, Niv realised that what the voice said about robots not being built for recreation was true. Blank panelling covered their crotch, and once the robotification of the flesh beneath was complete, there wouldn't be anything between the kobold's legs but a metallic null patch.
The last of the plating finished covering Niv's body, and they soon felt the sensation of everything beneath being replaced, in a myriad of intricate and unknowable ways. Like the voice said, all organic parts of their body, every last bit of flesh and bone, was being replaced by far more powerful and efficient robotic equivalents.
The only thing left to convert was their head, and it seemed that the progress was taking on a more subtle approach here. A series of thousands of minuscule panels crawled up Niv's neck, merging and forming together into precisely positioned robotic faceplate pieces. Their snout was taken over by the artificial conversion, sealed shut as the maw inside underwent similar roboticisation. Their eyelids were forced shut as their eyes' artificial replacements formed above. As the cameras that would provide the creature with its sight came into being, the world was once again visible around them. The clarity of their new vision left Niv stunned, and what little of their head that was left to convert was quickly completed in similar fashion.
Niv blinked its mechanical eyelids over its glowing-green-irised eyes, and looked down at its body. It realised it was now thinking of itself differently, as more of a something than a someone, and it loved it. Its body was perfect, the orange metallic panels of its body forming a perfect robotic facsimile of its once-organic form. It curled its claws in its hands, knowing that it didn't need to test them to know just how sharp and powerful they now were. Even its horns were sharpened to a point, and made practically unbreakable.
"Excellent! I see your new body is complete! Well, I'm sure you're going to enjoy being one of us, and, also..." As the voice paused, the device still attached to the back of its neck dropped into the dirt below, "Well, you know what to do next!"
It lifted the heavy organic conversion device as if it weighed nothing, and spotted a vixen jogging in its direction in the distance using its powerful new vision. As usual, the voice was correct: It knew exactly what to do.