In context: Historically, Diablo games have wound up having seven playable classes, with the first in the series being the exception. Typically it has been five in the core game and two additional coming with expansions. Diablo 2 added the Assassin and Druid in the Lord of Destruction pack, and D3 added the Crusader with the Reaper of Souls and the Necromancer with Rise of the Necromancer.
Diablo 4 is off to the standard start with five classes – Barbarian, Druid, Necromancer, Rogue, and Sorcerer – leaving room for speculation on which characters could wind up on the roster in future updates. Although Diablo games have been a mixed bag of previously used classes, Blizzard introduced unique characters in both mainline sequels. The Amazon and Witch Doctor only appear in D2 and D3, respectively.
There now seems to be circumstantial evidence that Blizzard might introduce an entirely new class to Diablo 4 – the Blood Knight.
WARNING: What follows could contain mild spoilers, so turn back now if you'd prefer to discover this information organically.
Over the weekend, a player who goes by Thunderclaww stumbled upon a page of game lore titled "Crown & Fang of Fernam" (above). The passage tells of a centuries-old lord of a region called Gea Kul, who was obsessed with finding a way to become immortal. His fixation was so overwhelming that he turned to studying vampirism. He had his mages conduct dark experiments to extract the secret of vampiric immortality without the adverse effects of the curse, but all attempts failed.
His undoing was when he had his army capture a vampire, which escaped, killing him and all his soldiers but one. This sole survivor, Fernam, was turned, but a mage conducted a ritual to save his humanity. It was somewhat successful, and Fernam became the first Blood Knight. Others like him were created through the same dark technique, creating a "secret brotherhood." However, the clan had to remain hidden because of their unnatural origins.
The missive also mentions a crown and spear, which were mainly symbolic but held magical or cursed powers. These could very well become cosmetic items sold in the store, while the character class described could be part of one of the two announced upcoming expansions or perhaps another further down the line. After all, it does seem slightly premature to introduce new playable characters this early in the game's life.
However, this is pure speculation at this point. Not only has there not been an official announcement for new classes, but no other evidence has been found through game lore or data mining to corroborate the existence of a Blood Knight class in Diablo 4. That said, data miners did find male and female Blood Knight models and animations in Diablo Immortal, but no playable class has emerged there yet.
At this point, there are a couple of ways to view the current information. The first is that the Blood Knight is a possible upcoming character for Immortal, and the lore page found in Diablo 4 is nothing more than an Easter egg tie-in. The second viewpoint is that the Blood Knight was intended for Immortal, but Blizzard was unsatisfied with it and cut it from the game until it could utilize better hardware to make the new class look and perform its best.
Since there has already been modeling work done and a reasonably decent fleshed-out backstory, it's hard to see developers just abandoning it. Players are already clamoring about D4's lack of a holy class. Diablo 2 had the Palidin, and D3 and Immortal had the Crusader. The Blood Knight sounds more like an unholy class, but it could still fill that warrior mage itch.