These are large, mummified creatures in the Warcraft universe that look like a mix between humans and spiders, and are members of the Nerubian spiders. Their lower bodies are supported by six spindly arachnid limbs and their heavily bandaged torsos are vaguely humanoid in form, though their heads are more generally spider-like. The Crypt Fiends were once the dwellers of the ancient spider kingdom of Azjol-Nerub that spanned the entirety of Northrend in ancient times. However, the Nerubians’ power base was destroyed by the Lich King, Ner’zhul, when he and his undead Scourge took control of the frozen continent. The former Nerubians, revived in undeath and now known as Crypt Fiends, command their spider underlings for the sake of their master, Ner’zhul. These aberrant creatures are capable of summoning swarms of vile insects and projecting web-like strands to incapacitate and harm their enemies.

When Sylvannas broke free of the Lich King’s control and freed a large number of undead this apparently did not include any crypt fiends (and crypt lords for that matter) since they aren’t seen in any of the Forsaken settlements in World of Warcraft. No explanation is given, but it is possible that the Lich King simply has more control over the hive-minded Nerubians than over more free-willed Humans, or that most humans were turned into scourge troops through plague, while the Nerubians were raised through necromancy.