Black Abishai

(From Mordenkainen's Tome of Foes (2018):  

Expert assassins and infiltrators, black abishais can weave shadows to mask their presence, allowing them to reach a location from where they can deliver a fatal strike to their targets.

Abishai:

Each abishai was once a mortal who somehow won Tiamat’s favor before death and, as a reward, found its soul transformed into a hideous devil to serve at her pleasure in the Nine Hells.

Emissaries of Doom. Tiamat deploys abishais as emissaries, sending them to represent her interests in the Hells and across the multiverse. Some have simple tasks, such as delivering a message to cultists or taking charge of worshipers to carry out a sensitive mission. Others have greater responsibilities, such as leading large groups, assassinating targets, and serving in armies. In all cases, abishais are fanatically loyalty to Tiamat, ready to lay down their lives if needed.

Outsiders in Hell. Abishais stand outside the normal hierarchy of the Nine Hells, having their own chain of command and ultimately answering to Tiamat (and Asmodeus, when the dark lord chooses to use them). Other archdevils can command abishais to work for them, but most archdevils do so rarely, since it is never clear whether an abishai follows Tiamat’s orders or Asmodeus’s. There is inherent risk in countermanding an order given by Tiamat, but interfering with Asmodeus’s plans invites certain destruction.

(From Planescape: Monstrous Compendium Appendix I (1994):

Abishai are common to the first and second layers of Baator. They look like gothic gargoyles, thin and reptilian, with long, prehensile tails and large wings. The three varieties of abishai have different skin colors — in ascending order of station, black, green, and red. All have a vinegary smell and rasping voices. The air seems to warm perceptibly in their presence.

Abishai are voracious and evil. They delight in tormenting those few baatezu lower in station than themselves. Abishai tempt mortals bold enough to travel to Baator by using their change self and charm person abilities to impersonate more powerful baatezu.

The abishai make up large, evil armies that fight the tanar’ri and intruders into Baator. In some cases, a red abishai may prove worthy enough to command a force of lemures. If successful, the red abishai may be promoted to a higher form of baatezu.

As part of their efforts to corrupt mortals, abishai like to bestow powerful magic on inexperienced wizards. Usually the low-level spellcaster cannot control these enormous energies, and chaos and destruction result.

Combat: In battle, the abishai strikes with two claws (1d4 points of damage each) and its flexible tail (ld4 + 1 points of damage and poison; note that the poison is fatal unless a successful save vs. poison is made).

Abishai can fly high into the air and dive at their enemies, striking with both claws. They attack at +2, and a hit does double damage (2d4 points per claw).

In addition to the powers of all baatezu, an abishai can change self, command, produce flame, pyrotechnics, and scare.

Once per day they can attempt to gate in 2 to 12 lemures (60% chance of success) or 1 to 3 abishai (30% chance).

Abishai are susceptible to holy water (2d4 points of damage per vial). They regenerate 1 hit point per round unless the damage is done by holy water or a holy magical weapon.