Biomass is the main fuel of the various Cast abilities and a foodstuff when needed.
It is a bile-like fluid that is made of the physical essence that comprises of all living things.

Various creatures of the Cast are capable of taking living material (i.e flesh, plant matter) and breaking it down, converting it into biomass that can be used to fill their primordial pits and nurseries, making their "Cast system" possible and sustainable.

Whenever a Cast hive is first formed the first directive of its workers and warriors is to gather materials to be converted into Biomass.
Without it the Cast essentially cannot reproduce.

The Chitinous armored mound crowned with spines and spikes with many orifices through which the swarms enter and exit is only the head visible above the ground. The living building keeps its main body buried and protected deep underground where its internal brooding chambers hatch hundreds of Cast Larvae.
The Swarm Hive is similar to an anemone, it's anchored and might even be considered plantlike by the way it responds to stimulus.
The Hive is an advanced form of Cast Tool. It is created when a worker drone enters a chrysalis and transforms into the Hive.
Spawning Pools: The Swarm Hive has a cavernous body buried under the ground. It's internal organs are designed to produce huge quantities of Cast Larva through Chrysalis for the Swarm and also serves as a construct to house Swarm creatures. The Swarm Hive is immobile when it's producing larvae, but when the Swarm is moving it can uproot itself and burrow through the ground.

A living Cast weapon. A bug like creature with a long elastic segmented tail with sharp spine like joints that can interlock into a rigid sword shape.
A living chain sword.

The Plague of the Cast is an infestation of spores produced by Cast that have aestivated and devolved into flowers.
Living creatures who contract this plague become sick within a week and within two days are quickly bedridden. Depending on the situation it could then take three weeks to a few days for the viral strain to fully infest the living creature, creating an infested version of the creature.

In the beginning stages it manifests as a mild cough, but within two days it worsens until the victim is bedridden. During the bedridden stage the victim begins coughing up black bile as the viral strain begins to feed and reproduce inside the victims body. As it progresses the victim proceeds to the vomiting stage where they start regurgitating larger amounts of the black bile.
By the five day mark the victims will begin to exhibit black stains on their skin in splatter patterns wherever they cough or vomit on themselves. From there they begin to manifest spines and tendrils that grow and grow until...

Because it is specifically a living organism it's better classified as a parasite rather than a disease or poison. Regular healing magic can temporarily slow the infection process as it repairs the damage done to the creature, but healing magic will also heal the plague in the process, allowing it to propagate and redouble its infestation efforts to an even greater degree.
Healing potions, herbs, medicines and other normal or magical methods of curing diseases are treated as food to help the strain grow and quicken the process of infestation. But dark magics, negative healing and necrotic energy can kill off the plague for good.

Creatures infested by this plague (The Infested) are mindless monsters solely bent on killing non-infested creatures. However, they are not aimless. Out of any group of infested a leader will arise, the parasite/plague will differentiate between the infested hosts and determine which one is the strongest or most intelligent amongst the infested group, and a special adaptation will occur in this individual, giving it control over the rest of the infested.