Stalkers

Humanoid creatures with a hunched-over stance and a hard, bony exoskeleton, they appear almost like walking skeletons. Their heads feature four small, black eyes, with two large horns and a strange, beak-like mouth. Workers have a number of small spikes on their backs and elbows, with Queens featuring yet more spikes and a far larger overall size. At the ends of their arms are primitive, claw-like hands which can rip open many types of armour.

According to the Notix, before they encountered the Stalkers, their civilization extended through many star systems, and Icicle was just their most remote outpost. The Stalkers nearly exterminated the Notix, except for those on Icicle.

They're an alien species with no technology of their own, but they've spread across much of the galaxy by riding along with those unfortunate enough to encounter the Stalkers somewhere. They kill relentlessly, and their bony skin acts as a natural armour which allows them to survive all sorts of attacks.

They're considered a bit of an interstellar plague, but no-one knows where they originally come from.

One of the Ventronian motherships which survived the destruction of their empire at the hands of the Theleans was completely overrun by these things after an asteroid containing Stalker eggs collided with the ship. The entire ship was eventually lost.

Definitely carnivorous.

If it moves or if it shoots at them, the Stalkers are going to try to tear it apart regardless of whether or not they can eat it. Stalkers are definitely mortal, so given enough firepower/other deadly features, you can kill them.They're fast, armoured and absolutely relentless though, and their numbers tend to overwhelm their prey.

When one Stalker is spotted that means that there are many more nearby. Though they tend to sit still rather than wander about or rather, hang still. They have a thing for hanging from ceilings.

They're not likely to take over entire planets - they don't appear in large enough groups for that - but wherever they land, they cause great chaos. An asteroid filled with Stalkers collided with that mothership at some point, and caused the final demise of the Ventronians. Their mothership, while heavily armed and inhabited by tens, maybe hundreds of thousands, was unable to stop the Stalkers from advancing and wiping out pretty much the entire ship.

The Stalkers are much more intelligent than pretty much any animal, and are able to use logic to find solutions for problems, and can sometimes recognize, understand and replicate speech as well, but they have no technology of their own (they never needed tools in their evolutionary path). They are full-on predators who simply aren't able to feel anything for anything other than their own kin, and so diplomacy/reasoning would be wasted on them. So while they are highly intelligent, they just don't have the capacity to form a culture/nation of their own.

With enough work, one may be able to train a Stalker to work for them, but they'll always have to watch their back: it'll likely turn on them if it ever believes that to be in its own best interests, and it's unlikely any amount of training/torture is going to change that.

They have extremely hard natural armour, are incredibly fast, and know how to use the environment to their advantage.

They have sharp claws that can cut through most standard armour, and are quite strong physically too, able to lift heavy objects with relative ease. Their intelligence allows them to abuse some of the systems on board spaceships, like sabotaging fire safety systems to create diversions, or ripping out control panels and using the electricity to their advantage.

They might fall into the traps the first couple of times, but once they understand that the environment is working against them, they'll start anticipating traps and working carefully to avoid or abuse them.

The effectiveness really depends on the environment.

The Stalkers don't usually fare so well on planets - too much open space - but in confined spaces like spaceships and spacestations, they are incredibly difficult to fight.

Stalkers kill because they need to eat, and because their instinct is simply to kill anything that moves. They can spend incredibly long stretches of time in a dormant state, staying perfectly still and consuming hardly any energy at all, so as soon as they detect a potential prey they wake up to try and eat again.

They'll still be able to detect breathing and body heat, but if the aliens have a suit that hides those things they can avoid detection. Once they make a move and Stalkers identify them as a target, they won't be able to lose their attention by standing still though. If they have no way of identifying them as a living creature, they'll ignore them. But even a rock creature could be seen as a threat if it moves, and while they won't be able to eat it, they'll still try to kill it. To the Stalkers, pretty much everything is either prey, or a threat. In either case, they'll try to kill whatever it might be.

Vaash will kill a Stalker pretty effectively, but if they figure out where the alien is storing it they'll rip open the containers and try to coat the user in it instead.

They'll leave trees alone as trees don't react to them. They aren't seen as living creatures, just like they'd ignore a piece of cloth moving with the wind.

As for where they came from, no-one really knows, though there's a few theories. They likely spread across the galaxy after some unfortunate souls landed on their homeworld and took Stalkers with them when they left. Those Stalkers may have spread to other ships and spacestations that way, and could have transferred to asteroids that hit a space station. It's also possible they had invaded spacestations that were blown up to try and exterminate them, causing fragments of whatever asteroid that station was built on to fly off into the void of space. It's also possible they were put their intentionally, but none of the modern races know exactly how they spread.

The Stalkers have an incredibly hard exoskeleton that looks a lot like chitin. They can actually survive out in the vacuum of space, shutting down all functions to the point where they appear more dead than alive, only to wake up again if they detect a threat. They can be dormant for decades without any issues, which has allowed them to spread the way they have. Enough brute force will generally do the job - they have strong armour, but a railgun will still easily punch through it. Just hitting them is often quite an issue by itself: they can move surprisingly fast, and know very well how to use cover. They can crawl along ceiling and walls too, so you always have to stay alert when Stalkers are near.

An individual Stalker can be quite troublesome, but when they appear in groups they really become a great threat. They also fare better in tight corridors than large open spaces, since they tend to use the environment to their advantage. They'll still come at their target at full speed, but they won't have anywhere to hide making them easier targets. If there's two or more of them, they'll coordinate to strike from multiple angles, but they lose some of their advantages when they're fully exposed with nowhere to hide.

They could dig holes and trenches, giving them cover as well as good places for ambushes, but they aren't built for burrowing. They could dig a hole and bury themselves, then jump out when they detect motion above, too.

If all they have to go on is vibrations in the ground, they may jump out, but as soon as they realize there's a friend above they'll stop their assault. It's unlikely they'd ever run into such a situation though, since they coordinate where they hide with others. Any Stalkers who haven't buried themselves will know where not to tread. Their footsteps are quite light, and the specific patterns may tell them that there's another Stalker above the ground, but whether or not they can use that sort of identification depends on the type of soil they buried themselves in.

They use multiple senses for communication and disrupting all of them may be difficult.

The Stalkers did come from a single planet, though no-one's ever been able to discover their birth world. Whatever planet it was, it was a very dangerous world. Resources there were scarce - which is why the Stalkers evolved their extreme form of hibernation - and temperature fluctuations were pretty extreme. It's likely that radiation was pretty high there as well, given the Stalkers' resilience against radiation. How the Stalkers were transferred onto asteroids is unknown as well. They don't have technology of their own and wouldn't have been able to leave their world without alien intervention, but exactly who's responsible for unleashing them onto the galaxy is unknown.

The Stalkers' origin and history is largely unknown. Their first major appearance was a very catastrophic one, as it spelled the final doom of the Ventrons. The Ventrons have long ago been driven from their homeworld and all of their colonies after they lost their war with the Theleans. Over the years, as more and more of their kind were hunted down in retribution for their near-destruction of Thelea, the Ventrons became a rare sight in the galaxy. They had one fleet left, composed primarily of their massive mothership and some smaller vessels.

The exact circumstances of the final incident are vague, but at one point a stray asteroid struck the Ventronian mothership. That asteroid was filled with dormant Stalkers, who quickly awakened and spread throughout the mothership. The narrow corridors and maze-like internal structure of the Ventronian mothership made it impossible to fight off the Stalker threat, and eventually the entire ship was killed off save for those few who managed to board an escape pod.

After that incident, the Stalkers have shown up from time to time all over the galaxy, always either through an asteroid or a derelict vessel filled with dormant Stalkers. Thus far, no-one has been able to find the planet these creatures come from, nor has it been determined how these Stalkers came to inhabit stray asteroids. So yeah, essentially, they just happened one day.

The Stalkers don't have a true hive-mind, though they do seem to communicate and coordinate with one another. They have queens, who are responsible for laying the eggs that hatch into more Stalkers, but the queen isn't quite as vital to their colonies as most Stalkers can eventually grow into a queen, though the process is unclear. There have never been any records of colonies with multiple queens, or even of any partially grown queens, but colonies that were confirmed to not have a queen have been found to reproduce and did have a queen in later inspections. Studying these creatures is extremely dangerous though, so knowledge is scarce.

Stalkers are highly intelligent, and have been noted to use strategy to ambush and confuse their prey. They work well alone but are even more dangerous in groups, and while they are extremely violent, there doesn't appear to be any malicious intent behind them. They simply kill to feed, and instinctively try to kill anything that moves and which isn't another Stalker. After they've killed their prey and had their meal, they tend to go back into hibernation, and they've been shown to survive extreme circumstances during their hibernation (such as zero-G environments, complete lack of oxygen, food and water for untold lengths of time, freezing temperatures and lethal radiation levels).

A Stalker encounter is actually one of the things Aegonian was hoping to add as a space encounter. (In Starbound mod)