Theory: The Loa Mueh'zala is an Infinite Dragon
Mueh'zala's only appearance, in game or in novels, has been in Zul'farrak in Tanaris. His name is mentioned on the Tablet of Theka in the Zul'farrak dungeon, and he is actually summoned by the Sandfury Trolls in "Traveler: The Spiral Path". Coincidentally, the Caverns of Time, the home of the Bronze Dragonflight and the place where we see most interference by the Infinite Dragonflight, is also located in Tanaris.
On the Tablet of Theka, Mueh'zala is given a series of different titles: God of Death, Father of Sleep, Son of Time, the Night's Friend. All of these can be explained, either figuratively or literally, if Mueh'zala is an Infinite Dragon.
God of Death. The Infinite Dragonflight is obsessed with bringing about the Hour of Twilight, an event set into motion by the Old Gods to bring about the destruction of all life on the planet. As an Infinite Dragon, Mueh'zala would not only be incredibly powerful (able to kill your average mortal with little difficulty), but would also be a harbinger of planetary destruction.
Father of Sleep. This seems like the most figurative of his titles, and can refer to the fact that death is sometimes referred to as "the big sleep", and that the Farraki Trolls have some sort of religious and literary tradition that links the two. On this same track, because an Infinite Dragonflight Mueh'zala is trying to bring the "Hour of Twilight" into this world, he would be a father of "the big sleep".
Son of Time. This one is pretty obvious. As an Infinite Dragon, he would have formerly been a part of the Bronze Dragonflight and thus sired by Nozdormu, the Dragon Aspect of Time.
The Night's Friend. Again, this title seems more figurative, but makes some sense with relatively little extrapolation. Infinite Dragon Mueh'zala would be trying to bring about the "Hour of Twilight", and the period following the actual hour of twilight is night. He is the Night's Friend by ushering in the Night (the destruction of the world).
According to the Warcraft book "Traveler: The Spiral Path", Mueh'zala takes many forms, including a twelve foot tall troll, a giant lizard, a human, a burning black flame creature, a floating whale shark, and a red-rimmed specter of black. Normally, however, he is seen as a huge indistinct shadow of constantly changing shape.
In the Warcraft universe, no dragonflight except maybe the Black Dragonflight is more famous for shapeshifting illusions than the Infinite Dragonflight, as that is their main tactic when trying to disrupt timelines in the Caverns of Time. This easily explains why Mueh'zala would appear as a human or a giant Troll, depending on which group he was trying to manipulate at the time. A giant lizard is a no brainer, because what are dragons but giant lizards? A creature of burning black flame would also describe just about any member of the Infinite Dragonflight. The most peculiar descriptions however are a floating whale shark and a red-rimmed specter of black, but given that dragons can take just about any shape they wish, both of these descriptions are relatively reasonable.
Additionally, an ever-shifting shadow that has no constant shape in space is an interesting concept for a creature that is so often exposed to the magic that produces and separates different timelines. Because of this exposure, the creature is constantly shifting in and out of our dimension, thus not all of it can be seen at once unless it decides to appear. A being without shape but seemingly limitless forms sounds pretty "infinite" to me.
In "Traveler", Mueh'zala is preparing to consume the captives of the Sandfury tribe when he is stopped by Aram. After this, Mueh'zala says:
"Not yet, Son of Thorne. Not yet. This is not the day. The day comes. It comes. But Mueh'zala will not engage you here or now. Our battle is yet to come, yet to come... But it will come, child. It will come. And if you lose that battle, Mueh'zala feasts on all of Azeroth. All of Azeroth. All of Azeroth. All of Azeroth..."
This death that comes to all of Azeroth reflects the motives of the Infinite Dragonflight to bring about the Hour of Twilight, a presumed inevitability indicated by "The day comes. It comes." After saying this to Aram, Mueh'zala sinks into the sands, perhaps acknowledging the strong sand imagery with time and the Bronze Dragonflight or to rejoin the Old Gods trapped beneath the surface. Or just because it's a desert, idk.
So yeah, where's Mueh'dormu, Blizz?
Sources:
https://wow.gamepedia.com/Mueh%27zala
https://wow.gamepedia.com/Tablet_of_Theka
https://wow.gamepedia.com/Infinite_dragonflight
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