The Prajñā Group (般若衆, Hannyashū; English TV "Hannya Black Ops") is a regiment of the Anbu of the Land of Woods.
In the anime, about six years after the Nine-Tails' Attack on Konoha, the village moved for a potential alliance with the Land of Woods, agreeing to trade special scrolls as a sign of trust. The Prajñā Group was assigned to go to the meeting point. Ultimately, the Prajñā Group betrayed the Konona-nin, attempting to kill them after receiving the enemy's scroll. Their attempt was thwarted by a Konoha Anbu team appointed by Danzō Shimura of Root and all but annihilating the Prajñā Group. Nine years later, on his way to the Kage Summit, the remnants of this group emerged in the hope of exacting revenge against Danzō. In spite of his old age, Danzō responded by killing every single member of the group, resulting in the Prajñā Group being wiped out forever.
Prajñā is a Buddhist concept, often translated as wisdom. There are many interpretations of Prajñā, but it usually refers to the wisdom that is able to extinguish afflictions and bring about enlightenment.
In the anime, the sword-length kunai used by these Anbu were replaced with regular kunai and swords.
Prajñā is also a translation of the word "hannya", which is a mask used in noh theatre to represent a jealous female demon. This is in reference to the design of the mask being worn by all the members.
This is further emphasised in the anime when the group's leader is shown to be a woman who calls the rest of the team with a demonic laugh.