The novel received generally positive reviews, winning the award for the 2012 Senior Irish Children's Book of the Year.

Just as Artemis leaves his final session of therapy for Atlantis Complex, he and Butler are summoned by Holly to the office of Commander Trouble Kelp, where they witness the next plan of Opal Koboi; she has two of her underlings kill her past self, who had survived the Kraken blast in The Time Paradox.

During her stay in the nuclear reactor, Opal furthers her ability to use black magic and opens The Berserker's Gate, a portal located on the Fowl estate, behind which dwell the spirits of fairy soldiers killed in the Battle of Taillte, an ancient war fought nearly ten thousand years previously.

When Artemis, Holly, and Butler arrive on the estate after a last-minute escape from Haven, they immediately try to foil Opal's plan, but they end up in hiding with Mulch Diggums, who was trying to rob Fowl Manor.

Important to the plan..." Colfer stated that he wanted the novel to deal with Artemis' transformation "from being a selfish criminal to a hero who is prepared to sacrifice everything for a good cause.

Kirkus Reviews praised the book, writing: "Colfer pits his resourceful crew against an army of killer bunnies and decomposed corpses (most of the estate's other residents being off for Christmas).

"[8] Another positive review came from Philippine Daily Inquirer's Ruel De Vera, who wrote: "Eoin Colfer's 'Artemis Fowl and the Last Guardian' is a most fitting final caper for the ever-planning young genius who brought the readers along with him as he went for mere smart bad person into something much, much more.