Artemis Fowl Book 5 - The Lost Colony

Ten thousand years ago, humans and fairies fought a great battle for the
magical island of Ireland. When it became clear to the fairy families
that they could never win, they decided to move their civilization
underground and keep themselves hidden from the humans. All the fairy
families agreed on this, except the 8th family, the demons.
The demons planned to lift their small island out of time until they had
regrouped and were ready to wage war on the humans once more. However
the time spell went wrong, and the island of Hybras was catapulted into
Limbo, where it has remained for ten thousand years.
Now, the tainted time spell is deteriorating and demons are being sucked
back into the present space and time. The fairy Council is naturally
concerned about this and is monitoring any materializations. When the
spell's deterioration accelerates, the materializations become
unpredictable. Even the fairy scientists cannot figure out where the
next demon will pop up.
But someone can. Artemis Fowl, teenage criminal mastermind has solved
temporal equations that no normal human should be intelligent enough to
understand. So when a confused and frightened demon pops up in a
Sicilian theatre, Artemis Fowl is there to meet him. Unfortunately, he
is not the only one. A second, mysterious party has also solved the
temporal equations, and has managed to abduct the demon before Artemis
can secure him.
This is a disaster for the fairy people, as this demon was no ordinary
fairy. He was the last demon warlock, the one who held the key to the
survival of the entire demon race.
It is up to Artemis and his old comrade, Captain Holly Short, to track
down the missing demon and rescue him, before the time spell dissolves
completely and the lost demon colony returns violently to Earth.