As of the end of Necropolis, it is still currently unknown where the Old Ones came from, how long they have existed for or even what their motives for attempting to wipe out humanity are but considering their Lovecraftian characteristics it is likely that such things are beyond human comprehension.
The Old Ones comprise of an army of evil creatures of large variety:
- Fly-soldiers. There are millions of large flies in the army of The Old Ones, which seem to have a collective consciousness. They fly long distances in swarms, but can form the shape of a soldier when on the ground. They have an advantage as any attacks can pass virtually straight through them, but their swords are sharp and their shields hard as metal.
- Knights. There are many knights on horses in the army, and the knights wear completely black armour that is covered all over in poisonous spines. The horses are also monstrosities, with poisonous spines, and some with swords impaled in their heads to give them a horn.
- Shape-changers. There are various combinations of animals twisted and merged to create monstrous hybrids, often between a human and another animal. The shape-changers can appear as humans, and talk and interact, but can transform into their hybrid forms for battle.
- Slaves. The humans who joined the Old Ones had to fight on the front line of their army, both in the present and the past. In the present, the leaders of the slaves were previous NightRise executives who were "adjusted" at camps - limbs replaced by blades, permanent iron masks attached, and various other adjustments to rid them of humanity and drive them insane.
- Fire Riders. There were thirteen Fire Riders in the army. Fire Riders are cloaked men who ride horses, and would accompany Chaos into battle. They could immediately burn and disintegrate anyone with the lightest of touches.
- Giant animals. As depicted in the Nazca Lines, the Old Ones had four giant animals in their army - a Spider, Condor, Hummingbird and Monkey. The animals were absolutely massive, making them deadly against any human. However, they were not invincible - in the battle in the past, the monkey was taken down by flaming artillery.
- Chaos. Also known as the King of the Old Ones, Chaos was the leader of the Army. He appeared in different ways. When he escaped from the Nazca Gate in Peru, he was an enormous black giant, who towered over Matt - his fingernails were taller than Matt himself. In the battles in NightRise and Oblivion, he appeared as the shadow or silhouette of a man, but twice as tall. He had an outline, but no features. Raven's Gate and the Gate of Oblivion were formed by the Five all stabbing Chaos simultaneously with swords to create a five-pointed star when their sword tips touched, banishing the entire army into the 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.
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.
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.
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.
In Oblivion, time had moved on ten years since the events of Necropolis. The Old Ones had changed the world drastically - famine and poverty was widespread in the USA, slavery was abundant in South America, governments collapsed worldwide, terrorism had crippled Britain, and 'natural' disasters occured like Vesuvius erupting in Italy. Over-population, terrorism, climate change, fascism and corruption had affected the entire world.
The army of the Old Ones had long since abandoned Hong Kong to Oblivion, Antarctica, to an ice fortress built into the mountains. Most of the army was at Oblivion, however, there were several units spread across the world: shape-changers aided the government in Egypt to capture Scarlett; fly-soldiers aided the police in England to capture Jamie; shape-changers and the giant Spider guarded the Door at St Meredith's Church in London.
Shape-changers helped to run the fortress at Oblivion - they watched over the slaves, and guarded the fortress, including the secret passage in from Skua Bay. Fly-soldiers captured Matt when Scott tricked him into a meeting at Skua Bay, and the giant Condor picked off any aid from the World Army as Matt was captured.
When the World Army launched missiles at the fortress, it appeared destroyed. However, when the infantry reached the fortress, it was revealed to be a trick, and the full army came out of the fortress to massacre them. They would have been entirely slaughtered had Matt not split the entire ice shelf in half. Once the Five were all inside the fortress, at the Cave, the full force of the Old Ones exploded out of the mountain, including Chaos. However, Chaos had been distracted by his revenge on Matt for injuring him, as the Door was unlocked, allowing Pedro and Jamie to be there; Scarlett had sneaked in using the secret passage; and Matt and Flint from the past had travelled to the future to replace the dead Matt and Scott.
After a short battle, the Five united in front of Chaos, and all stabbed him with their swords, the tips touching. Chaos, and the whole army of the Old Ones, were sucked into a portal, and the Gate of Oblivion was formed, which would hold forever, probably sealing the Old Ones out of Earth for eternity.