The Shape Changer is a minor, yet important antagonist in the fourth Power of Five book Necropolis by Anthony Horowitz and he plays an important role in the story. He is actually one of the Old Ones in disguise.
Origins
Being one of the Old Ones, this demon came to Earth ten thousand years ago and tried to kill off humanity with his master Chaos. However, they failed. He may have been one of those who killed Sapling, Jamie's original incarnation, but this is undisclosed. Jamie recognizes him as a Shape Changer the moment he meets him, which implies the Shape Changer may have fought in the First War. Its likely he did as he was one of the Old Ones. He got banished when the Five met and killed Chaos the first time in the past. However, he broke out of Hell through the efforts of Diego Salamanda, who used dark magic and astronomy to free the Old Ones on June twenty-sixth 2006 and the demons broke out of the Nazca Desert.
This Shape Changer presumably watched Chaos punch Matt into a coma and then fled with Chaos into the night to Antarctica where Chaos made his headquarters in an ice palace named Oblivion.
Biography
Role in Necropolis
When Richard Cole, Matt and Jamie are heading to Heathrow Airport in London to meet Scarlett, to tell her of her destiny, they have put so much effort and planning (which includes being funded by the Nexus and getting illegally through immigration with Senator John Trelawney) into finding her in London which seems crazy to stop right now. As is the case with the best laid plans, this does happen.
Right by Dulwich, in South London, a taxi shoots out just as the Jaguar the chauffeur is driving them in is accelerating due to being cooped in slow London traffic. The taxi slams out of a drive and crashes right into the Jaguar, wounding the driver who gets a broken arm. Everyone else gets whiplash but otherwise they're fine. When Richard stumbles out, not listening properly from the pain, he knows something is wrong when the Shape Changer stumbles out of his taxi and profusely apologizes for smashing into them. Richard accuses the Shape Changer of deliberately smashing at them but he denies it saying it was an utter accident and he wasn't looking where he was going.
Matt also sees the Shape Changer, and suddenly Jamie, who also stumbles out, sees that the man is one of the Old Ones, a Shape Changer. Matt believes him and suddenly sees how the man's skin is so pink it looks fake and how the body clings to him like an old coat. Matt believes Jamie because Jamie met Shape Changers himself back in the past. He threatens to expose the Shape Changer's true form (said to be an alligator) to everyone in the street but Richard warns him against it. If the Shape Changer was angered, they had everything to lose, having fake passports. So, knowing they have to let it go easily, Matt says nothing and then policemen arrive. The creature pretends its glad to see them and Matt can see his voice is as fake as the rest of him. When the Shape Changer talks with the police saying he's glad they're here he is ordered to breathe into the breathalyzer and this makes Matt smile because he knows the Shape Changer's breath isn't human and if he overdosed on alcohol its unlikely it would register. Then the Shape Changer is last seen being accompanied by policemen when Richard, Matt and Jamie are briefly taken to the station, given tea, then released.
Afterwards
When London was ordered to be destroyed by Chaos, the Shape Changer may have gone back to Oblivion to start the war or he may have stayed behind in the ruins of London to hunt down Jamie or the other Gatekeepers if they return.