Lei Shen

Lei Shen, better known as the Thunder King, was the first Emperor of the Mogu in Pandaria when the various clans were unified under his reign. This signaled the end of the Age of a Hundred Kings and the Thunder King reappeared before the Siege of Orgrimmar, prompting the Alliance and Horde to loosely band together to halt the Mogu Empire from being recreated.

History:
According to Zhi the Harmonious---Caretaker of the Golden Lotus---the Thunder King was an emperor named Lei Shen and various weapons and armor of the Thunder King were buried in shrines and tombs around the Vale of Eternal Blossoms. The Thunder King was a tyrannical Mogu warlord who united the Mogu factions at some point in the distant past and became the first emperor of the mogu in Pandaria.

The Thunder King rose to power “the mogu way”; namely, by eliminating his rivals one by one. According to legend, he was able to accomplish this with the help of a mysterious artifact that he found in the depths of a mountain. After his rise to power, he built the Mogu'shan Vaults to contain and protect this secret and converted the surrounding structure into a sort of shrine dedicated to the mogu empire.

(( Adapted from wowwiki.com/Lei_Shen ))

As commonly heard by members of both Factions, Lorewalker Cho of the Golden Lotus stated the following in regards to the Thunder King:

The mogu tell a legend about Lei Shen, the Thunder King.

''It is said that he tore out the very heart of the Mogu God. And from that hateful act he gained power over wind and storm. His followers fell to their knees before him. "We will call you 'The Lightning King'," they said. But Lei Shen did not agree.  "Lightning strikes in an instant and is over in a flash," he said. "But, thunder. Thunder! Thunder proclaims the coming of the storm. Thunder quakes the skies long before the lightning strikes, and thunder echoes in the hills long after lightning's power is spent. It is thunder that sends animals cowering and fills the hearts of peasants with dread. Let thunder be my herald, so that my power is felt throughout the land."  "I will be... The Thunder King."''

''Oh my friend - that awful sound. For a hundred-hundred generations the crash of thunder filled the hearts of pandaren with terror.''

It is said that after the revolution, the Red Crane of Hope placed the rainbow in the sky, so that Pandaren would know not to fear the storms any longer.

Legacy:
To the Mogu, Lei Shen is revered and each Emperor or King succeeding would attempt to relive his legacy of subjugation, torture, domination and building of great, ornate structures.

To the Pandaren, Lei Shen is synonymous with horrible acts, atrocities, the beginning of their slavery and fear of thunder itself.

All races (except the Mantid) were enslaved by Lei Shen and during his time, the Serpent's Spine was built along with various other great structures---such as the Mogu'shan Vaults, Mogu'shan Palace, and the Isle of Thunder stronghold.