Typhon

Typhon (or Typhoeus) (Τυφών in Ancient Greek is the last child of Gaia/Gaea. He is also known as the "Storm Giant" or "Father of all Monsters." His wife is Echidna, and his father is Tartarus. Any time that Typhon was defeated, thousands of venti were born. He was truly a bane of Olympus as stated by Chiron.

History
Thousands of years ago, after the First Olympian War, Gaia was enraged at the defeat of the Titans, her children and of how they were locked away in Tartarus. She attempted revenge by sending her last and most powerful child, the great monster Typhon to destroy the Gods. Typhon alone is the greatest foe the Gods ever faced, even mightier than the Titans and the Giants themselves.

Having never anticipated such a rival, the Gods were quickly forced on the defensive. Zeus, however, struck Typhon square in the chest with the Master Bolt at point blank range, greatly weakening him. The Gods were then able to trap him in Mt. Etna, where his raw power itself causes lava to ignite from the mountain top in the form of a volcano. After the defeat of her most powerful child, Gaia admitted defeat and allowed the Gods to rule Olympus without interference.

Western Civilization
When the Gods moved and followed the center of power of Western Civilization, Typhon was trapped in Mount St. Helens. Like in Mount Etna, his raw power ignites lava from the mountain top. Though he was successfully imprisoned for thousands of years, his prison could not hold him forever. During his efforts to kill a gang of Telekhines in The Battle of the Labyrinth, Percy was forced to call on all of his power to escape Mount St. Helens. It was this that weakened the seal on Typhon's prison and awakened the Father of all Monsters. It was soon that Typhon would rise again to finish what he started at the First Olympian War or the Titanomachy - destroy the Gods.

The Last Olympian
Typhon escapes in a catastrophic explosion of Mount St. Helens. Following his escape, Typhon heads to New York. Using the Mist, Typhon is able to conceal his true form from the mortals and appears as a freak storm. The Gods, at the very least Artemis, Apollo, Athena, Hermes, Ares, Dionysus, Hephaestus and Zeus are the only ones named, are unable defeat him as Poseidon is occupied defending his undersea kingdom and Hades, Demeter and Persephone refuse to fight. Not even Zeus's Master bolt, the most powerful weapon ever created, could stop him (Percy, through a projection on Olympus, saw that he he merely stumbled backwards before continuing his way towards Olympus). While the Gods are busy fending off Typhon, the Titans made their way to Mount Olympus, which was being defended by demigods.

Percy prays to his dad and asks him to leave his kingdom and come to help the Olympians defeat Typhon. His father agrees and when Typhon steps into the Hudson River, the call of the ocean is heard and Poseidon rides into battle along with Tyson (who was 10 meters tall) and his brethren, the Cyclopes. They chain him, and Poseidon opens a tunnel to Tartarus, and casts Typhon down. Once Typhon was defeated, a wave of ventus (storm spirits) were born.

The Lost Hero
Jason Grace, Piper McLean and Leo Valdez went to Boreas to help them in their quest to save Hera, and as he explained the release of the storm spirits, he showed them a video image of the gods defeating Typhon with Poseidon's help.

Appearance
Typhon's true appearence is unknown and perhaps impossible for a mortal or even a demigod to view without going insane. Through the mist, Typhon appears to mortals as a massive storm and tornadoes that tears apart everything in their path. To people who can see through the mist, his shape is constantly changing to every monster and horror one can imagine. If a demigod or mortal looks at him too long they risk going insane. Based on comments he has some sort of structure to serve as legs (possible serpents according to myth) and something to serve as claws or hands to attack his enemies.

Powers
Typhon is one of if not the most powerful being in all of Greek mythology. He rivals the combined powers of most if not all the major Olympian gods. The full extent of his power is unknown. He can create storms of tremendous power including tornadoes and possibly hurricanes. He has unbelievable physical strength and durability surpassing that of any individual god. He was able to fight most of the major gods to a near standstill, withstanding all of their attacks throughout including the Master bolt, numerous lesser thunderbolts, the arrows of Apollo and Artemis, and numerous other attacks. He was strong enough to hit Hephaestus with enough force to create a new lake and Dionysus deep into a mountain. He presumably has the standard powers of a god, but surpassing that of any other.