Amnizu

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

Amnizus lead the infernal legions into battle and command guardians at the gateways to the Hells. Amnizus are arrogant, bullying, and ruthless, but they’re also highly intelligent tacticians and unfailingly loyal — qualities that the hellish archdukes value.

Guarding the River Styx. Some amnizus perform the critical task of watching over the River Styx from fortresses along the river’s blighted banks, where it flows through Dis and Stygia. Souls arriving in the form of lemures have no personalities or memories; they’re driven only by the desire to commit evil. The amnizus that patrol here drill the rules of the Nine Hells into the new arrivals’ pitiful brains and marshal the lemures into legions.

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

The amnizu, as greater baatezu, enjoy a nobility of sorts. They follow orders only because of their desire to advance and their fear of the pit fiends. The higher a fiend advances in Baator, the more closely the Dark Eight watch it. Given the opportunity, however, amnizu order their minions to attack anything under the control of the Dark Eight. Possibly an amnizu would even attack a pit fiend given sufficient reason, if it thought it could evade suspicion.

The amnizu guard the river Styx, which enters Baator through Stygia as well as Avernus — a weak point in Baator’s defenses. For their own perverse ends as well as the protection of Stygia, amnizu individually command an army of thousands of abishai and erinyes.

The amnizu protect Baator from invaders, and thus they are given their power of imprisonment. Mortals who travel here to combat evil generally become fodder for the amnizu.

The amnizu dwell primarily on Stygia, the fifth plane of Baator. They often enter the upper layers of Baator on missions.

Combat: The amnizu are greater baatezu and, as such, prefer to leave combat to underlings. Their touch, a channeling of harmful energy, does 2d4 points of damage. It bypasses ordinary armor, so the amnizu need only hit Armor Class 10 to cause damage. Magical pluses of armor, shield, or a protective device such as a ring of protection improve the defender’s Armor Class. Magical items that give a base Armor Class also protect against the touch.

Because of the amnizu’s close ties to the river Styx, its attacks work as a powerful forget spell that causes the target to forget one whole day’s memory unless he saves vs. spells.

In addition to those magical abilities available to all baatezu, an amnizu can use the following spell-like abilities, one per round: fireball (3 times per day) and imprisonment (once per day). Once per day amnizu can attempt to gate in 2 to 20 abishai (50% chance of success) or 1 to 8 erinyes (30% chance).

A holy word drives an amnizu back to its own plane, Stygia.