Space bug castes overview

The Ruin are essentially interchangeable with something like the Tyranids in Warhammer 40,000. Unlike the Tyranids, they have characters because that is more interesting to write about when writing an RTS campaign. 

Myriad

"We are Myriad. Our numbers shall darken the skies of every world. In the end, we shall prevail. All other considerations are trivial."

The Myriad is the gestalt consciousness of the Ruin. If every individual drone within the Swarm may be considered neurons that collectively form a brain, then the Myriad may be considered the higher functions of that brain. The Myriad is an alien mind to humanity, devoted to nothing more than conquering the universe and perfecting the Ruin into the ultimate form of life. They are the ultimate alchemists.

For all intents and purposes, the Myriad is the Ruin. It derives all its desires from its constituent drones. Despite this, it has abstracted itself to the point where fawns over its drones like a parent does offspring, referring to them as "my children" when it formulates orders and directives. Likewise, the Ruin look to it like children do a parent... or more accurately, a god.

The Myriad is responsible for creating the player character and setting them on their journey.

Strategists

"Enemy detected. Moving to intercept. Feast!"

"Belay that order."

"Do not intercept? Do not feast?"

"They are no threat to us yet. Keep sensors trained on them."

Directly below the Myriad in the hierarchy are the Strategists, who serve a similar role to commander characters in human hierarchies. Strategists are responsible for overseeing battles and expositing to the player character, among other responsibilities.

Like the Myriad, the Strategists are not physical entities like humans are. They are artificial intelligences created by the Myriad to oversee lineages, pursue directives, and breed their lineage to pursue that directive more efficiently. Just as the Myriad is the sum of all the Ruin, a Strategist is the sum of its lineage.

Indeed, Strategists are so specialized for their directives that they generally cannot operate efficiently by themselves. Like the armed forces of human nationstates, a given invasion swarm may actually be composed of multiple Strategists working together. One might be responsible for mining raw materials, another for spawning warriors and defending the hives, another for leading attack waves on enemy forces, etc. 

The player character in a Ruin campaign is a Strategist, as is usually the rest of the cast.

Drones

"Hunger, feast, eradicate, learn, evolve..."

The living organisms that compose the Ruin are known collectively as drones. Although there are countless different castes, there are a handful of specific ranks that castes generally fall into. These are Synapse Drones, Menial Drones, Complex Drones, and Brain Drones.

Synapse Drones are bred to directly coordinate other drones on the battlefield or in non-combat situations. They are at least as intelligent as humans in terms of raw output and readily adapt to changing conditions, although they are born with a specific skillset and cannot easily deviate from that anymore than humans can change their college major after decades of experience. They are technically capable of communication with other species, though they never do so willingly. Castes include those bred to be frontline commanders, those bred to oversee the spawning and maturation of drones, and those bred to infiltrate enemy worlds and subvert them from within.

Menial Drones are bred for menial tasks, such as mining, construction, and combat. Individually, they are single-minded and will follow their last orders received until they die or receive new orders. Normally they are coordinated by Synapse Drones, but this is done for efficiency. In the absence of Synapse Drone, Menial Drones will network their brains in order to behave more intelligently at the cost of efficiency elsewhere. It is a dangerous mistake to underestimate them as merely feral animals.

Complex Drones are bred for tasks that require more intelligence than a Menial Drone, but more specialization than a Synapse drone. They serve roles such as scientists, engineers, deep space probes, and espionage agents. Complex Drones of certain castes may diligently carry out their tasks for long periods without contacting other drones. Some may have limited synapse capacity and be capable of controlling symbiotic menial drones.

Brain Drones are bred to oversee tasks performed on a planetary scale, such as strip mining, mass production, or wholesale warfare. Their intelligence and ability to process information is far beyond any individual human. As the name suggests, they are little more than gigantic brains ranging in size from a school bus to a continent. Depending on their caste, they may more closely resemble animals or plants in basic morphology.

That's all for now. Hope you enjoyed!