This is a sequel to "Artemis Fowl and the House Elf," a cross-over with Harry Potter. There are spoilers for AF:TP and HP:DH.
Chapter 1: A Call from Artemis to Artemis
"Abraxas Faire speaking," said Artemis Fowl, answering his cell phone in his new Dublin flat.
His older temporal counterpart had suggested the new first name, the grandfather of the young wizard the other Artemis impersonated in a parallel world. Artemis had come back from a trip to Limbo to discover that three years had elapsed and that his place had been taken by another Artemis from an alternate time stream. It was hard not to resent this, but he and his counterpart had come to an arrangement such that the original Artemis had a new identity and a chance to work with his "brother" on interesting schemes.
"This is Artemis," said the other Artemis. "I need your assistance desperately. Mother is deathly ill, dying, and the doctor has no idea how to cure her."
"I assume you attempted a healing of your own?" Abraxas asked.
"Naturally, but it only seemed to make her worse," Artemis replied.
"You know a bit more magic than I do," said Abraxas. They both knew this was an understatement. Artemis had spent several months in intense magical training at Hogwarts, a top wizarding school in the parallel universe.
"True, but because of that the Faerie trust me less," Artemis answered. "I need your assistance to persuade one or both Holly Shorts to help Mother." (There was also a duplicate Holly Short, whom the alternate Artemis had saved from a life of slavery at Hogwarts).
"Of course I'll help," Abraxas replied. "She's my mother, too."
Suddenly a strange voice broke in on the conversation from the Fowl Manor side: "Young man, drop that muggle instrument of communication."
Abraxas heard the phone hitting the floor, but he could still hear everything because Artemis had the clever impulse to press the speaker-phone button before he dropped it.
"Professor Dumbledore? What are you doing here?" Artemis asked in a shocked voice.
"You recognize me," said Dumbledore. "Good. That means you are the time interloper I'm looking for."
"You seem angry," said Artemis. "Don't you realize that I saved you from a plot against your life? Professor Snape..."
"Was a valuable double-agent working for me," Dumbledore interrupted. "Your interference has disrupted my plans at a vital time. It is only fair that I make use of your impersonation skills to replace him."
"But my mother is dying!" Artemis cried.
"The fate of my world is at stake," said Dumbledore. "You are coming with me at once."
Abraxas heard a burst of static over the line.
"Artemis? Artemis?" he called.
There was only silence in reply.