The Geolith

The Geolith comes in many different forms, much like its Elemental and Golem relatives. Descended from ancient Tool-Golems used to build fortresses both below and above ground, The common Geolith ancestor is to blame for many of the ruins that dot the underground. Nowadays, the Geolith has been split into 4 different families, each differing from the other in often drastic ways.
== Aquarx ==
The Aquarx Geolith resides deep underground, usually around the level of diamonds. It attaches itself to the walls and floors of caves, latching on to undead that walk over them, and eating them alive. (or dead?) The geolith is very easily defeated by functioning immune systems and is more of an annoyance for miners due to its resemblance to diamonds. Like most geoliths, they are sentient beings and can communicate via weak psychic signals. One can form a contract with a geolith, which has many health benefits. With the Aquarx, it can filter one's blood, preventing disease and poisoning (though sadly, this also applies to alcoholic drinks...) The Aquarx will also do everything in its power to strengthen its host, allowing one to live a healthy life for longer. The Aquarx is not a solution for immortality, however, and it's betting on your death. 
== Crimsai ==
The only geolith native to the nether, and one out of the two types of geolith that can overwhelm the living immune system, the Crimsai resembles the red stone of its homeland embued with quartz. Much like its counterparts in the overworld, the Crimsai naturally feeds off of the roaming undead present within the nether, shooting quartz-like harpoons to make it so the undead can't move away. This of course triggers the rest of the horde to swarm the Crimsai, but due to its unique body structure, this is little issue for the Geolith. The Crimsai is all about strengthening its host to allow it to survive in the harsh conditions of the nether. Most commonly found bonded (or even subsumed) in the brute class of piglin, its abilities are not to be underestimated. Crimsai tend to have more agressive personalities than their overworld counterparts, save for one. Because it resembles exposed bone and meat, the Crimsai is one of the least sought after for aesthetic reasons, as its bonded form can look rather gruesome on a human (or otherwise sentient race.
== Vacol ==
A rare Geolith, The Vacol can technically reside anywhere. However, it is most commonly found within Lush caves, masquerading as moss growth. An easy way to tell Vacol from moss is that Vacol still feels like stone, while moss is softer. The Vacol is unfortunately unable to hitch a ride on even the weakest undead, and can only spread via a symbiosis with nearby plants. When bonded with a host, the Vacol resembles a greenish-grey moss. It filters blood, like its cousin the Aquarx, however it can also promote coagulation and wound healing at an impressive rate, to the degree where wounds can be mended in real-time. The Vacol is intelligent in this process and prevents things like bones from fusing back together incorrectly. It also clears one's skin! 
The Vacol is highly sought after by nobles, as its ability to increase their survival rates against assassinations, while also improving their complexion, is a steal of a deal.
== Voidalai ==
Otherwise known as the "Tyrant Ink" The Voidalai is a mindless, primordial, and debatably downright evil geolith variant. It is behind many "farmers turned rampant kings" stories. The Voidalai itself is found at the bottom of the world, resembling bedrock. It doesn't stay there long, however, as it hitches a ride from the bottom of the world to the surface, perhaps over billions of years in some cases. Unlike the other geolith, the Voidalai lacks a mind. Because of this, it cannot form contracts, which would be a problem if it needed them. It is the other Geolith that can very easily overwhelm the living immune system. Unlike its hellborne counterpart and its cousin variants, It instantly kills and subsumes itself within the host. Oddly, this doesn't seem to affect the host at first. The newly formed Geolisk now has a mind for the first time in its life, and it's nearly an exact copy of the person before, unlike other geolisks, who tend to be completely different from their host. There is one key difference between the old host and the new Geolisk, and thats is the fact that the Voidalai geolisk, now given a mind, is very, very greedy. They can finally experience the world with their own hands, and they want to have it all to themselves. It's theorized that many of history's tyrant Kings were voidalai geolisks.
== Geolisk ==
The contract ends when the host expires, which is when the geolith takes over. Substituting the blood of the host with itself, the Geolith and the host body meld together to become a Geolisk. The Geolisk looks similar to the original host in every way, other than a change in blood color from the functional red (or otherwise colored blood) to the geolisk color. For example, an Aquarx Geolisk has teal blood. From that point onward, the Geolisk can move freely and act freely. Geolisk are technically classified as a monster and are treated as such in many nations. Their ability to easily blend in makes it hard to persecute them, however. 
Your best friend could be a geolisk, and you probably wouldn't be able to tell. Though, Geolisk don't age anymore, so if you've been friends with someone for 80 years and they still look like they're 24, it's probably a geolisk...
== Common Features ==
All Geolith symbiotes (and their Geolisk counterparts) have a few things in common. The most common being the gift of eternal youth, but not eternal life. If, in the unlikely case one discovers and acquires eternal life, any symbiotic contract is considered void, and the geolith will leave its host, often through self-termination. 
Given enough nutrients and time, a geolith can revert to its common ancestor, A Geocore, and will begin the construction of a dungeon. Different geolisks only respond to certain races however. This can make commanding a Geocore difficult if you are not of said race. Aquarx prefer short lived races (Max 150 years in lifespan), and The Vacol prefers longer-lived races (+150), but neither respond to Demonic or hellish races. The Crimsai will, however. 
The Voidalai seems to only respond to itself, and godly powers. This, along with its lack of a mind, makes it the hardest to control. 
=== Trivia ===
* The Geoliths (and Geolisks) are common knowledge monsters, much like zombies or skeletons. One can know of these things even without meeting one in person via books, or word of mouth. The intricacies of a Geocore are unknown, however.
* Geocores will build a "church" around themselves, which serves as the heart of a dungeon.
* Geocores will build above-ground "spore spires" to draw in undead to feed themselves. These spores, upon contact with the living dead, will gain control over the corpse. They cannot resurrect regular corpses, however. Wild geocores may use this to their advantage, using hordes of undead to protect and fuel themselves.
* Only fully grown Geocores can be commanded. The other variants (Geoliths and Geolisks) are free-willed. This does not mean that the Core will not attempt to defend its own life if endangered.
* Everything within a Geocore dungeon is grown from the geocore. This is why they cannot fabricate food, as it may look and feel like a food item, but in reality, it is mere stone.