Kalran: Tales of the four nations/Phoenix gas

The rat cell of the Reclaimers are rather infamous for their use of toxins and their willingness to field test it, whether at the enemy on the field, innocents from a town, or even their own clones. Ranging from nerve gas to metal melting acid, but none more a toxin loathed then the phoenix gas.

Derived from the cordyceps fungus, aided with copious amount of enchantments, the phoenix gas is best described as a gas holding onto trillions of manufactured toxin spores.

Once a person is exposed to the gas, the spores will begin to invade, finding any bare flesh to take root or digging its way through thin materials, upon which it will begin to infect and kill the host, compromising the host immune system and poisons their cells. And once the host has reached critical conditions the spores insidious purpose is revealed.

From there the spores will begin to invade and take control of the host nervous system and the brain, if its intact, and begin forcing the body to move to its will, where upon which it will attack nearby creatures to further spread it spores and intake nutrition's needed to sustain itself, and thank to being man made the Reclaimers had also implanted nanobots that allows them to control the infected host. During this if the user is unfortunately alive will have full conscious, but little to no control of their infested body, as they can only watch in horror and beg for help. Scarily this also works against ai, controlling the machines just as well as a being while converting any nutrients it takes to power the machine battery

The only saving grace that restrict such a horrid weapon is it's expensive process to create, taking a few seconds to disperse to uselessness once exposed to air, can be quickly treated in the early stage, and that the infested host tends to only live for a few hours, unless constantly fed, before the fungus runs out of nutrition's and begins to disintegrate.

Mk2 gas
Another saving grace was that the spores have a hard time penetrating thick armor, making vehicles immune to the gas but forcing the crew to button up tends to hamper their effectiveness in combat, with the crew only getting infected if their armor is breached. At least that was until Mary had made a horrific breakthrough. As she found out that, with some modification, the gas can penetrate thicker barriers.

This modified gas now has the spores now learning how to slip through the little gaps or break through the thinnest part of the vehicle, once that's done it will infest the closest being, and the infested will attack any remaining crew. And once all the vehicle crew is dead the spores will begin to fuse its host with the rest of the crew and their vehicle, transforming into a twisted creature fused to metal.

Fortunately the process to make the improved version is even more expensive while the gas is even harder to contain, requiring expensive/specialized containers. Thus only high ranking commanders can only access the improved version if they're in fierce combat.