In light of the news of Patch 5.2, let's take a look at who the Thunder King is. Lei Shen was the first ruler of the mogu, the first to unite his people. In a very real sense he was the first Emperor of what is today Pandaria. He cleverly united the mogu people not merely through force of arms or even powerful magic, but through motivation. He learned the lessons of strength by studying the mantid, and applied the lessons he learned to his own people.
It has been written that when the great first looked upon the lands of the mantid, he did not feel fear, but inspiration.
As he began to unify his people under a single banner and subjugate the other races of Pandaria, he knew that the mantid would never succumb to his authority. They spoke his language, the language of strength. He commanded his slaves to construct the Serpent's Spine, a magnificent wall that spanned the length of the empire.
It would take many generations to build. But Lei Shen knew how to motivate his subjects. Fear. Fear of the mantid moved mountains, raised armies, secured his empire, and built his wall.
The Thunder King, from Dark Heart of the Mogu
Before he could apply the lessons of the mantid, however, Lei Shen would first need to rise to power, which he did with the aid of an ancient lost artifact. This artifact allowed Lei Shen to annihilate his rivals and cement his power over the mogu, and the Thunder King is said to have built the Mogu'shan Vaults to contain this weapon.
The King of Thunder
We do not know where Lei Shen was born, how long it took him to cement his power over the rival mogu clans. We know that he did so in the 'mogu way', which means he killed them one at a time, and he did so with the aid of the 'mysterious artifact' from the Mogu'shan Vaults. It was Lei Shen who ordered the creation of the Vaults, although whether he actually built them or simply repurposed them is unknown -- it's clear that many of the secrets of the vaults are Titan in origin, such as Elegon and the Creation Forge at the heart of the Vaults. It's easy to imagine Lei Shen using that same Forge to create an army of warriors to destroy his enemies.
During his long reign Lei Shen subjugated the rest of the races of what is today Pandaria, bringing the jinyu, pandaren and hozen under his control. He crafted edicts that were in essence the very first laws ever recorded in Pandaria, dealing harsh punishments like public evisceration to anyone who dared rebel against his rule. But despite being a brutal conqueror, Lei Shen was also pragmatic and wise enough to recognize when conquest would not be feasible or achieve his aims. He looked upon the mantid and saw them as an exemplar, a strong people who would never yield to his rule or serve his ambitions directly, and so he made them serve him indirectly, by using the fear and suspicion his people felt towards their kind to unite them in hostility and terror, and use that to motivate them to erect the Serpent's Spine across the land, forever separating the mantid from the fertile lands he sought to rule.
He spoke the language of strength
He was also wise enough to realize that the ancient Troll empire of the Zandalari, which was itself engaged in a vast war with the insect people known as the aqir, could serve as a useful ally. The two peoples were separated enough that conflict wasn't immediately necessary, and shared similar outlooks on life and the exercise of power, and their alliance became close enough that when the eventual pandaren rebellions happened long after Lei Shen's death, the Zandalari fought alongside their mogu allies to suppress it.
We also know that it was during the reign of the Thunder King (the 170th year of said reign) that the Korune clan created the Divine Bell. Said bell was crafted by the Korune from 'the flesh of the makers' and it was capable of instilling great hatred and violence into the soldiers of Lei Shen's army - making them raging juggernauts, engines of raw destructive power.
Eventually, however, even Lei Shen could hold death back no longer. Upon his death, his remains were enshrined in the Valley of Emperors, named such because his successors took to burying themselves near him. The Tomb of Conquerors in the valley held Lei Shen and his army of terracotta warriors, while his artifact helm was enshrined in the Gou-Lai Halls in the Vale of Eternal Blossoms so his people could come and see it and be inspired by its presence. His legendary axe and spear were likewise preserved.
Interestingly, Lei Shen entrusted secrets of potent magic to his Zandalari allies, and recently said Zandalar at last found Pandaria again after having been driven away during the war the pandaren call the Zandalari Troll Wars. In the confusion following the parting of the mists that kept Pandaria hidden for 10,000 years the Zandalari found Lei Shen's body and, using those ancient magical secrets, brought him back to life.
With Xin the Weaponmaster dead at the hands of invaders to the Mogu'shan Palace, Lei Shen is the ruler of all mogu in name as well as fact, rightful king of all his people once again. And now, with the news of what's coming in patch 5.2, we'll see what Lei Shen has in store for all the misbegotten rebels of Pandaria.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.