Typhon

Typhon is the last known child of the earth goddess Gaea, and is also known as the "Storm Giant" or "Father of all Monsters." His wife is Echidna and his father is the spirit that is Tartarus. He is especially hated by Aeolus because when he is defeated, thousands of Venti are released thus making the wind god's job vastly more difficult. He is among the most powerful foes and one of the largest threats to Olympus.

History
Thousands of years ago, after the First Olympian War, Gaea 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 themselves.

Having never anticipated such a rival, the gods were quickly forced on the defensive and they eventually went into hiding. Athena, however, stood her ground and convinced Zeus to fight by calling him a coward. This convinced Zeus and he went into battle with his lightning. Zeus 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 under Mount 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, Gaea admitted defeat and fell back to sleep, allowing 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 Saint 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 Jackson was forced to call on all of his power to escape Mount Saint Helens. It was this that weakened the seal on Typhon's prison and awakened the Father of All Monsters. Typhon would soon rise again to finish what he started after the First Olympian War; destroying the gods.

The Titan's Curse
When hypothesizing about the identity of the "Bane of Olympus", Chiron suggests it might be Typhon.

The Battle of the Labyrinth
Typhon is mentioned by Hephaestus. Later, Percy causes an explosion in Mount Saint Helens that makes Typhon stir in his sleep.

The Last Olympian
Typhon escapes in a catastrophic explosion of Mount Saint 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, or at the very least Artemis, Apollo, Athena, Hermes, Ares, Dionysus, Hephaestus, and Zeus, are unable defeat him as Poseidon was occupied defending his undersea kingdom from Oceanus and Hades, Demeter, and Persephone refused to fight.

Not even Zeus' Master bolt, the most powerful weapon ever created, is able to stop him, as shown when Percy, through a projection on Olympus, sees that Typhon merely stumbled backwards before continuing his way towards Olympus. Typhon also incapacitates Dionysus and Hephaestus along the way, knocking Dionysus into a mountain range, and causing Hephaestus to create a new lake in West Virginia. While the gods are busy fending off Typhon, the Titans make their way to Olympus, which is defended by demigods.

Percy prays to his father and asks him to leave his kingdom and come to help the Olympians defeat Typhon. His father agrees and reluctantly leaves his kingdom to assist the gods. When Typhon steps into the Hudson River, the call of the ocean is heard and Poseidon rides into battle along with Tyson and his brethren, the Cyclopes. They chain the monster down, and Poseidon opens a tunnel to Tartarus, casting Typhon down. Once Typhon is defeated, a wave of venti (storm spirits) is born.

The Lost Hero
Jason Grace, Piper McLean, and Leo Valdez go to Boreas to get help for their quest to save Hera. As the god explains the release of the storm spirits, he shows them a video image of the gods defeating Typhon with Poseidon's help.

Appearance
Typhon's true appearance is unknown, although in the Greek myths he is described as having a hundred dragon heads sprouting from his shoulders and neck. Through the mist, Typhon appears to mortals as a massive freak storm and tornadoes that tear apart everything in their path. To demigods (who are immune or highly resistant to the Mist) he appears as a large shadowy figure compose of dark clouds. If a demigod or mortal looks at him too long they risk going insane from trying to comprehend what they are looking at. Based on comments, he has some sort of structure to serve as legs (possibly serpents according to myth) and claw-like structures or hands to attack his enemies. Percy also says that he could not identify the monster's face because it changes every second to an even more horrifying monster than the previous one.

Abilities
Typhon is one of the most powerful beings 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, storms, 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 multiple attacks from 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 smash Dionysus deep into a mountain. He presumably has the standard powers of a god, but surpassing that of any other.

Trivia

 * Typhon is often known as the most powerful monster of all, but Porphyrion has claimed that he and his fellow giants are most powerful children of Gaea, which raises questions over who is the more powerful being.
 * Sphinx is the only monster in the series whose mother is Echidna but was not fathered by Typhon.