the old ones
The Old Ones are a group of creatures that the Five will one day battle in the present day. They were defeated for the first time by the Gatekeepers when they were all together 10,000 years before. The Old Ones are led by a creature called Chaos, who first appears in Raven's Gate as the primary antagonist, alongside Sir Michael Marsh and Jayne Deverill, who were working for Chaos. Chaos was the cause of the near-destruction of Earth before the Dark Ages where Jamie Tyler is catapulted to in Nightrise, where he meets the Old Ones in their previous forms. The forms that the Old Ones take towards the end of Evil Star are deformed and monstrous creatures which centre mainly on the creatures in the Nazca Lines in Peru. The Old Ones are capable of finding people who share the same views as them and giving them extra powers and plans for world destruction. However, at the end it is revealed that those people cannot destroy the world themselves - they have to die with the rest of humanity, because humanity is doomed and the Old Ones are the true masters of destruction. The Old Ones make an appearance in both Evil Star and Nightrise, although in Nightrise their presence is mainly due to the flashback wherein the Gatekeepers eradicate them. Despite these brief appearances, the Old Ones are the primary antagonists in The Power of Five, and their influence is felt throughout the series.
Raven's gate
The Old Ones first appear in Chapter 15 of Raven's Gate, mentioned by Professor Sanjay Dravid of the Nexus. The Nexus has been created to combat the Old Ones, and make sure the world remains safer whilst the Old Ones are kept in their prison. The only Old One to make a physical appearance in this book is Chaos, who attempts to escape from Raven's Gate with the assistance of the witches of Lesser Malling and the human antagonist, Sir Michael Marsh. Chaos kills Marsh when he comes out of his prison, taking the form of a huge black creature with webbed fingers. Chaos is then pulled back into Raven's Gate by radiation when the nuclear reactor beneath the Gate explodes and then he is not seen again until the events of Evil Star, where another Gate is opening, this time in Peru.
Evil Star
The Old Ones' presence is felt throughout Evil Star, but only at the end are they actually seen in physical form. The four main creatures in the Old Ones' army are a giant hummingbird, a massive condor, a giant spider, and a huge monkey. There are also thirteen fire riders who form the Old Ones' cavalry, and several other deformed creatures: a huge toad with the head of a bird,an alligator on legs,a pig the size of a horse, and several strange, ethereal creatures. The Old Ones escape from their prison and meet two Gatekeepers, Pedro and Matt Freeman. Pedro is unable to confront them due to an injury cause by a plane crash. Matt tries to fend off the Old Ones, and almost wins, but he realizes that he doesn't have enough power for he is one of the Five and all of them are needed to defeat the Old Ones. Matt falls unconscious allowing the Old Ones to escape led by Chaos. Just like the Old Ones possessed two giant dogs in Raven's Gate, in this book they possess several giant condors, which guard the Gate in the Nazca Desert, thus linking with the Inca's prophecy: The birds fly where they should not fly.
Night rise
In Nightrise, the Old Ones have escaped. They are assisting Nightrise Corporation in order to find the Gatekeepers, and Nightrise capture two: Jamie and Scott Tyler. Diego Salamanda has failed to capture Matt and Pedro, and Nightrise is also trying to look for them, and also the fifth Gatekeeper, Scarlett Adams, appears in the past when Jamie Tyler is shot while escaping the prison Silent Creek, dying temporarily and going back to the past, where he replaces his previous incarnation, Sapling, in order to destroy the Old Ones. The Old Ones rule the world in the past, and they seem invincible, to the point of actually killing a Gatekeeper. However, they don't know that when a gatekeeper dies he is replaced by his future self, and so they ignore the fact that Jamie has replaced Sapling. A brief battle follows with the Old Ones in which the Gatekeepers meet up and defeat the Old Ones, expelling them to another dimension. After this, two Gates are built to keep them out: one in Peru and one in Yorkshire, where the first battle was fought. However, they break out in Evil Star, which Jamie sees in his dreams in Silent Creek Prison.
Necropolis
In Necropolis, the Old Ones have seized control of Hong Kong in China, as this is where the ultimate headquarters of the Nightrise Corporation is situated. From Hong Kong, the Old Ones are spreading a fascist government across the world, commencing their new empire. Anyone who criticizes the government is eliminated, and anyone who informs people about the possession of Hong Kong also dies. In other words, the Old Ones have seized control of Hong Kong, and are possessing multitudes of its civilians, from the government to the police and the street cleaners. As the main plan of the Old Ones is to destroy humanity, they are slowly killing off all the surviving inhabitants of Hong Kong - the inhabitants who have not criticized the government or informed their relatives of the new regime - by infecting the atmosphere with a poisonous gas which resembles green acid, and this slowly sucks the life out of the living inhabitants. In Necropolis, the Old Ones have discovered the Fifth of the Gatekeepers: Scarlett Adams. They now need to trap her in Hong Kong, their city, and lure the other Gatekeepers into the same trap. This they succeed in doing, only to have the Gatekeepers escape at the last moment and Scarlett creating a typhoon which ultimately destroys Hong Kong, the Necropolis. Once the Old Ones have separated the Gatekeepers, they leave Hong Kong, presumably to Antarctica, where Chaos, the King of the Old Ones, has made his base in an ice palace.