Heart Leech

These so called Heart Leeches terrify me. That something so alien can adapt itself to our world so fast and so completely is abhorrent to me. I also can’t help but think, where did they come from and who so imprisoned them in the stars?

-Adeal Khan, 4th Grandmaster of the Imperial Knights

The strange parasite known as a Heart Leech is one of the many things the Emperor of the Atralum Empire considers secret. Their existence only revealed to a select few. These creatures are how he keeps his absolute control over the powerful Order of the Imperial Knights.

Origin

The first heart leech fell to this planet centuries ago, encased inside a metallic asteroid that the freezing temperatures of space had frozen solid. The descent through the atmosphere had thawed it out and the impact of it hitting the planet cracked open its celestial prison. It lay there in the impact crater, stunned and unused to being free. It had no concept of how long it had been stranded in the asteroid, and this planet was completely alien to it. Then a travelling merchant, sure he was going to find something worth selling, approached the pit where it lay. Thinking it was dead and that he might be able to sell it to a mage or scientist, he approached the leech. That’s when it struck, shooting forward and latching onto his throat. It cut into him and wrapped around his interior organs. He had collapsed in pain and shock at this unexpected assault; the leech used this time to sift through his memories. Looking for something it could use to further its survival.
When the merchant awoke, the leech imprinted ideas upon his mind, mentally suggesting he travel to the capital where it could try to insinuate itself with people of power. It was here a member of the Pyronate discovered the existence of the heart leech and brought it to the Emperor, seeing how it could be used.

Biology

The heart leech, when fully grown, is a multi segmented worm with a ring of serrated teeth and both ends. These mouthpieces function as more than just feeding parts. One end will latch onto the spinal cord, tapping into the mental and physical processes of its host. The other side will lock onto the heart. The longer it stays within a host, the more it integrates into their system. Changing them bit by bit until they are barely human inside anymore. If allowed to grow from a juvenile into an adult inside a single host, the heart leech gains even more control and power over their host. Changing them further than they could if implanted later.
Once joined to a host, it is exceptionally difficult to remove without killing both leech and host. Someone can only manage this if they catch it within the first month of implantation. If the host dies and the leech survives, however, it can move on to a new host as long as they are present at the time of death. The transfer must happen immediately after the host's death.

Powers

The leech is a sentient being with the ability for rational thought on or above the level of a human being. When implanted, if it requires, it can totally take over the host body and mind, but they have discovered that the leeches prefer to leave a mind intact to control the body while it lies largely dormant. Only wanting to take control if it needs to protect itself and its host.
If the host is unaware of the parasite, it can give mental suggestions imprinted into the host’s mind that the host believes are its own thoughts. They find themselves wanting to carry out these mental suggestions at significant risk to themselves.
If the host is aware of the parasite, then the leech will frequently communicate mentally with their host. Directly telling them its wishes. Often Knights find a level of equilibrium with their leech, being able to talk amiably with it. If not even having a form of friendship with it.
The bodies of long standing hosts acquire a long list of benefits they can enjoy. From harder skin that can withstand long falls or weapon cuts, to being able to go long periods without food or drink.
The leech can cause radical changes in the host body for short periods as well. Their main defence against potential destruction is to cause mouths to open over the host’s body and their skin becomes malleable, like a putty that can absorb enemies and devour them quickly. They can also do this same change on a smaller scale, opening a mouth within an appendage like a hand which can grip into a foe. If locked onto the foe’s spinal cord, the leech can access the brain and memories of their victim.