As Star Wars fans, we already know that there are several different kinds of stormtroopers, and the most recently seen branch are death troopers, as seen on the latest episode of The Mandalorian. If you're trying to remember what death troopers are, we've got some answers for you. The short version: these are not the guys you ever want to run into.
Prior to their appearance in The Mandalorian, death troopers were best known for their appearance in the Star Wars anthology film Rogue One, where they served under Orson Krennic and took part in the Battle of Scarif. According to Wookieepedia, death troopers are a specialised — and especially dangerous — branch of Imperial stormtroopers: "Death troopers were an elite variant of the Galactic Empire's stormtroopers designed for stealth, espionage and lethality. Operating under Imperial Intelligence, they served as protective details and bodyguards for significant Imperial officers and members of the Tarkin Initiative, as well as special-assignment commandos... Death troopers were trained in unarmed combat, heavy weapons, demolitions, improvised weaponry, guerrilla warfare, and marksmanship. They were experts at covering their tracks, leaving little to no evidence of their missions."
It's unknown, exactly, what became of death troopers once the Empire fell, but their reappearance in The Mandalorian does seem to give a few clues. They're seen in the further reaches of the galaxy now, presumably because Imperial remnants have been forced to flee from the main centre of the newly-freed galaxy. However, they're still clearly taking orders in the old Imperial command structure: in their appearance here, they answer to Moff Gideon, a former high-ranking Imperial official who now leads his own remnant army and who is seeking the Child for some mysterious (but certainly not good) purposes. The troopers are responsible for killing the Client, surrounding the Mandalorian and his allies, and protecting Moff Gideon and his scheme. As the first season of The Mandalorian hurtles towards its finale, we're pretty sure we'll be seeing more of these terrifying troopers soon.