The Cabals are an elite biotic turian unit whose dictum is, "The intangible is unstoppable." As turian biotics are relatively rare, with biotic abilities manifesting in only a small percentage of element-zero exposed turians, Cabals tend to be small and only comprised of 10 to 15 individuals led by a commanding officer called a kabalim.
A turian Cabal is often deployed covertly as shock troops, or saboteurs, or they're sent on long reconnaissance missions for the Hierarchy. In addition to advanced biotic training, all Cabal soldiers are highly skilled in small arms, explosives, infiltration tactics, and piloting.
Centuries ago, during the Unification Wars, military black ops would recruit turian biotics as infiltrators, planting them anonymously in the regular infantry as "observers" or using them as assassins. This historical practice has left a cultural bias against biotic soldiers among the turian rank-and-file. As a result, the Turian Hierarchy prefers to keep its Cabals isolated from other branches of the military, citing "safety and unit cohesion concerns." Even turians whose biotic abilities are not combat-proficient are segregated into Cabals, often forcing them to cut short their promising careers in order to serve as a medic or maintenance officer. However, Cabal soldiers are professionals; those who do not adapt are quickly weeded out, resulting in strongly unified teams of experienced, deadly, biotic warriors who see their isolation as a necessary sacrifice to protect the Hierarchy.
The war with the Reapers means that the Turian Hierarchy no longer has the luxury of keeping such powerful assets in reserve. Many Cabals have been recalled to Palaven, while the most experienced have been dispatched to the front lines for surgical strikes on Reaper strongholds or to provide support for regular infantry.