Tempestry began when a Necromancer named Kalen trapped Hydros at Carthax Lake using blackrock from the Pagan Obelisk. This was his vengeance, as Hydros had previously killed his beloved. Just as Kalen prepared to destroy Hydros, his hand was stayed when she promised the power of Tempestry to him and his successors. Thus they struck a pact where she would give her powers to his bloodline in return for keeping her alive, but in her deceitfulness she did not mention that she would take that power back should she be freed. The story of Kalen is documented in the manual:
Stilling the Seas
Many generations of hardships passed and a hero became known to the people. His name was Kalen, and in time he became their Necromancer. He fell in love with his Apprentice, who returned his affections, and soon they were to undergo the Ceremony of Bonding. Then tragedy befell them as Hydros, considerably displeased with the pact between Lithos and the Pagans, sent a great wave to wash across the city, pulling Kalen's beloved into the seas. As the people began repairs to the city, Kalen headed to the Temple of Flowing Waters.
There Kalen begged for the return of his betrothed, but Hydros refused his request. Instead she revealed to him the image of his love in her new form, pale and sickly as that of one of the Lurker's minions. Thoughts of vengeance entered Kalen's mind and he visited Lithos to learn of means for justice. He found the Mountain King eager to comply, having no love for a rival Titan. Lithos revealed that a powerful substance, called Blackrock, was immutable by any amount of the Lurker's powerful waves.
Kalen remembered the tales of the ancient Pagan Temple and the great obelisk. He returned to the volcano and found in the crater the remains of the obelisk. There, in the murky waters continually filled by Hydros' rains, was the darkened silt of this Blackrock. Using his Necromantic powers, Kalen reshaped the crater to prevent fresh water from entering the Temple of Flowing Water. In addition, he melded the Blackrock into the crater wall, preventing the Lurker from eroding away the land. He had trapped Hydros.
Entering the Temple, Kalen was prepared to destroy both it and Hydros by reforming the crater. His intention was that not enough water should remain for the Titan's existence. Before he could complete the task, the Lurker's pleas stayed his hand. She promised to return his beloved's body so that it could be interred properly, no longer a servant to the Titan of Water. She also offered to pass on to him and all in his future bloodline a modicum of her powers, equal in measure to the abilities he gained from Lithos. Kalen accepted her bargain for the good of the people. From that point on, the storms ended, and two separate sects of magic, Necromancy and Tempestry, were present in the land.
While the current Tempests are the lords of Tenebrae, this was a very recent development. The descendants of Kalen eventually moved to the fields and became farmers (as is documented in Bentic's Journal). Keldan son of Treal, one of his descendants, was eventually forced to marry Celidia, daughter of Lord Kean, ruler of Tenebrae; this is how the Tempestry moved to the royal family. Yet Keldan's love was elsewhere, and Devon was born from the love between him and Ariel. Ariel and her son were afterwards sent to live with the fishermen, and thus Devon is unconscious of his powers. Celidia eventually demanded a child from Keldan, and Mordea was born. Yet by order of birth, despite the fact that Mordea has taken the throne of Tenebrae, Devon is the rightful Tempest.
The powers of the Tempest are not learned but passed down from generation to generation of Kalen's descendants. Thus no reagents or foci are required in the casting, but none but the Tempest may wield them.
The Tempests have absolute control over water, and this includes weather. Tempests may manipulate the weather to their liking, cast lightning on those who assail them, and walk on water without harm to themselves.