The Swarm cast can communicate telepathically with each other and even see through each others senses for a hundred mile radius so long as one of the psionic hierarchy is present. They are thus able to act as a single creature with many bodies under the direction of the psionic. The Swarm Weird
has the benefit of hindsight, they are able to see the stored memories of other hive members who have failed before them and learn from their mistakes.
By consuming the flesh of a creature or consuming genetic material into their system their body will take only one of the most beneficial genetic codes from the material and assimilate that trait into themselves. They can then communicate this trait telepathically with the hivemind so that the entire swarm could use this trait to evolve. (Example: A cast consumes the meat of a Basilisk. The genetic essence is assimilated into the cast creature and they gain the ability to turn other creatures to stone on eye contact, or resist the effects of being turned to stone themselves, but rarely both. They send the construction of this genetic trait to the hivemind, where the trait could be replicated and added to the rest of the swarm should it prove useful.)
They are very biologically adaptive creatures. When placed in hostile environments such as fire or ice their bodies automatically begin adapting defenses against them, such as flame retardant secretions or heat shielding chitin, or quickly growing thicker fur coats or adapting their muscles to naturally create more heat when exposed to the cold.
Their senses are like that of dangerous predators, heightened senses of smell dominate their other senses, but they are capable hunters. The Weird is an animal, one with considerable cunning, intelligence, and dangerous natural weaponry that it uses to deadly effect.