The Stranger Things season 4 trailer introduced a terrifying new monster, unlike anything seen in the show before; and it's likely this is the creature referred to in episode titles as "Vecna." Every season of Stranger Things has upped the ante, with the Hawkins kids confronting threats of ever-increasing scale. The trailer for Stranger Things season 4 promises this trend will continue, introducing a monstrous new creature.
Unlike other monsters from the Upside-Down, this one is humanoid in appearance; it could well be the Upside-Down's equivalent of a human, or simply a person who was stranded in this dangerous realm some time ago and has been transformed. The monstrous being is shrouded in shadow, surrounded by sinewy tentacles reminiscent of the Mind-Flayer, and it seems to move with grim and inexorable purpose. Mirroring Stranger Things' Dungeons & Dragons themes, the dialogue over its first appearance ties to the board game idea; "You have lost."
Visually, this creature corresponds with a monster from Dungeons & Dragons lore named Vecna - a creature mentioned in the episode titles for Stranger Things season 4. In the game's lore, Vecna was a wizard who sought dark magic and gradually transformed himself into an immortal creature known as a lich; his worshipers further encouraged his ascension to become an actual deity. Vecna granted his lieutenant Kas a sentient blade to act as his agent, but it persuaded Kas to turn on his master, leading to Vecna's apparent destruction. He left behind only two magical artifacts - his hand and his left eye, imbued with powerful dark magic. Curiously, most of the lore surrounding Vecna was established after 1986, when Stranger Things season 4 is set, so it's difficult to say how closely the show's portrayal of Vecna will parallel Dungeons & Dragons; he'll presumably be named by the Hawkins kids based on the lore they know of in 1986.
The decision to feature a humanoid threat is an inspired one. Until now, the Hawkins kids have really been dealing with animals from the Upside-Down like the Demogorgon; the Mind-Flayer had an intelligence of its own, but it could only really act through a host. In contrast, a sentient being like Vecla can pose a real and tangible threat, weaving plans and schemes of his own that will force the children to adapt and improvise. He will not act on instinct alone, which means he isn't as easy to manipulate.
The interesting question is whether or not Stranger Things will indeed ascribe to the wider Dungeons & Dragons lore surrounding Vecna, by revealing this creature is in fact a human somehow transformed through exposure to the Upside-Down. Stranger Things appears to be rewriting the history of the Upside-Down, with an earlier teaser trailer focused on a haunted house associated with murders in the 1950s. The killer, a father named Victor Creel, claimed he was under the influence of some sort of demon possession when he murdered his children; Vecna certainly looks demonic, so it's possible it possesses powers of mind-control in a similar manner to the Mind-Flayer. If that is indeed an account of Vecna's past, then the creature has been trying to get through to this plane of existence for decades, and the Hawkins children may well be facing the Upside-Down's greatest threat in Stranger Things season 4.