Tempestry is the magic of water, fueled by the titan Hydros. The tempests are actually the rulers of Tenebrae, residing in the palace in the city centre. When threatened with death in ancient times, Hydros had to grant this power to the tempests. There are many powers. Tempests can walk over water, breathe under water, summon or dissipate storms, and call lightning bolts or explosions. Since all of it comes directly from Hydros, no words of power, reagents or other utensils are needed to do the magic.
Sadly, it is a inherited trait, and can't be learned, only granted by Hydros. In a attempt to do so, the Avatar made a deal and freed Hydros. But the hero was tricked, and the power of tempestry was lost for all time to Pagan.
From the The Chronicle of Pagan, "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.