Mual-tar
Mual-Tar, the Thunder Serpent is a Primordial introduced in Dungeons & Dragons 4e as part of the World Axis cosmology. He was the star of the first Lords of Chaos article in Dragon Magazine #370, which remains his only major source of information.
In appearance, Mual-Tar takes the form of an enormous serpentine form fashioned not from flesh but from storm-clouds and lightning. Great wings of blue fire spread from its back, thunder rolling in ear-bleeding booms whenever it flaps them, stirring the ever-present storm-clouds that shroud its form. Its head is unlike any mortal snake or dragon's; a weird snout with two prongs above and a third below, enveloping a circular mouth that resembles a lamprey's maw filled with rotating rings of jagged stone teeth. Its snake tail trails off into long, writhing tendrils, arcing with the lightning that crackles and surges from Mual-Tar's thrashing form.
This appearance is mimicked by Mual-Tar's "progeny", elemental monsters known as Primordial Thunderspawn, embodiments of the destructive power that emanates from Mual-Tar's body. These creatures resemble snakes, for the most part, save that their bodies are formed from solid stormclouds wreathed in lightning. They lack their progenitor's fiery wings or distinctive features; their maw is a gaping, cyclone-like vortex used to suck air and prey into their bodies. Vicious predatory beasts, they are often tamed by Mual-Tar's cultists.
In the modern era, Mual-Tar lies stranded atop a desolate plain of rock deep in the Elemental Chaos, lashed by eternal storms so massiove that they can be seen from 100 miles away. This continent-sized mass is the prison of Mual-Tar, site of his final battle. Five gargantuan stakes of the purest adamantine, forged by Moradin himself, are pounded deep into the stone, boring for untold depths into the hardest rock. Massive adamantine chains stretch from each stake, anchoring themselves in a great collar of ghostly rings and spikes that surrounds Mual-Tar's neck, a construction of magic to bind what no earthly metal could hope to hold.
Eons ago, during the Dawn War, Mual-Tar claimed dominion over the skies and the mountain peaks. This brought him into conflict with Moradin and Pelor, who had related areas of interest. These gods, with the aid of Bahamut - though none know just why the Platinum Dragon chose to battle this particular foe - battled the Thunder Serpent for ages. Finally, they cornered Mual-Tar in the depths of the Elemental Chaos, where Bahamut's strength, Pelor's magic and Moradin's artifice combined to chain down the Primordial, sealing it away.
The gods believed that their prison would Mual-Tar for all eternity. In this... they were wrong. Five hundred years ago, the maddened Primordial's struggles finally snapped one of its god-forged chains, sending waves of mighty storms, typhoons and hurricanes across the Elemental Chaos and into the World beyond. Since that day, primordial cults worshipping Mual-Tar have flourished, and that worship has slowly strengthened the Thunder Serpent.
Indeed, a second chain is now on the verge of breaking in turn. And if Mual-Tar's power grows with each shattered binding, then the loss of that second chain alone could mean catastrophe...
Facing Mual-Tar
It is said that, in the distant age long gone, Mual-Tar was a cunning and clever foe, easily as smart as any elder dragon. But eons of imprisonment and the sapping of its powers have driven Mual-Tar mad, causing it to descend into animalistic savagery. Whether being freed would restore Mual-Tar's mind, or merely unleash a frenzied beast with a power to make the gods quake, is unknown - and it's hard to say which possibility would be worse.
Were a brave band of adventurers to face it, they would be confronting an unrestrained, bestial force of nature at its most destructive. Though intelligent enough to use its Storm Strike and Wing Storm powers to keep from being surrounded, mostly, Mual-Tar focuses on the attack, lashing out with its thundering maw and electrical tail at everything around it.
Mual-Tar's Cults
Cults to the Thunder Serpent are concentrated in the Elemental Chaos, the World, and the Feywild. It is most popular with storm giants and their storm titan masters, but any race with an affection for the power of storms is willing to fall in with Mual-Tar. Humans, orcs, eladrin, goliaths and degenerate dwarves are some of the more common Thunder Serpent cultists, but there are also myriad yuan-ti worshippers as well - after all, those serpentfolk never met a snake-themed Power they wouldn't worship.
Because of Mual-Tar's mental degredation, cults to the Thunder Serpent are eclectic even by the standards of Primordial Cults in general, as their deity cannot hope to communicate its true desires or pass on real commands - some of these cults even see Mual-Tar more as a tool to exploit for power over thunder and lightning than an actual figure of worship.
The Hammer and Chisel Society: Meeting each month on the nights of the waning crescent moon, the hooded and masked members of the Society are a bunch of anarcho-hedonists who believe one should break free of the chains that bind people to certain expectations of behavior and social mores. Hidden behind their masks and hoods, the Society can cast away those morals and customs that restrict their freedom! ...Which ultimately boils down to partying themselves until they pass out, but hey. This may sound harmless, and ordinarily it would be, but as you've probably guess, the hedonistic front is only the outer layer; the inner circle are Mual-Tar worshippers, who take the Society's motto of "Break the chains!" all too literally, performing rites and blood sacrifices to help empower Mual-Tar so it may free itself.
Stormfury Orcs: Though most orcs are loyal to Gruumsh, the very nature of the God of Destruction's teachings leads many to supplement or replace this worship with reverence to Demon Princes and Primordials. The Stormfury Orcs are a tribe of plains-dwelling orcs who have chosen to venerate Mual-Tar instead of Gruumsh, which they express by gathering in massive armies whenever a thunderstorm brews upon the plains and run with it, attacking whatever settlements and homesteads the storm happens to batter in its wake. They can be distinguished by their practice of scarifying their faces and arms with lightning designs.
The Forest Temple of Sarpacala: Deep in a remote forest region prone to violent storms stands a ruined temple, dating back to the yuan-ti empire of Zannad. Still inhabited by descendants of the now-fallen empire, these serpentfolk have forsaken their racial god Zehir to instead venerate Mual-Tar, seeking his freedom through dark rituals that they believe weaken the Thunder Serpent's chains.
Eldresh, Bralani of Autumn Storms: This ambitious and greedy eladrin noble takes her title from her magical connection to the power of autumn storms, a rare variation of the more common Bralani of Autumn Winds. In truth, the source of her power is her worship of the Father of Storms, Mual-Tar itself. Unlike most cultists, Eldresh has no particular loyalty to the Primordial; she seeks Mual-Tar's power for her own goals, specifically overthrowing the ruling council of Mithrendain, and thinks that other cultists make useful allies towards this goal.
The Thunder Temple: In the depths of the Elemental Chaos stands the greatest temple to Mual-Tar that still exists, a palatial shrine that houses the most important and powerful cult of the Thunder Serpent known. Unfortunately, the three leaders of the cult have different visions of how best to lead, and their constant infighting and machinations leaves the cult acting erratically at best. Tarsharan is an ancient blue dragon who, having witnessed the wave of storms that occured when Mual-Tar's first chain broke, has become enamored of the storm's destructive potential; he wants to free Mual-Tar and unleash devastation upon all creation. Murandrae is an efreet karadjin who was exiled from her house for forsaking Imix in favor of Mual-Tar; she wishes to use the cult and the freed Primordial to conquer the City of Brass and make herself Calipha of all Efreeti - to this end, she is known to communicate with both the yuan-ti of Sarpacala and the Hammer & Chisel Society. Finally, Shar Malam is a Bluespawn Godslayer, a blue dragon-blooded abomination created by Tiamat of epic power and even greater ambition. This entity has no reverence for Mual-Tar at all; he wants to use the cult as a weapon for his own goal of destroying Bahamut and Tiamat both in order to take their place as God of Dragons. He considers the Stormfury Orcs to be something of a pet project, and often conjures storms to direct them to targets of his choosing.