Cahill Island is a small island appearing in Into the Gauntlet and Vespers Rising off the coast of Ireland. It was discovered by Madeleine "the Matriarch" Cahill (Gideon's great-great-grandmother). Gideon's grandfather built Cahill Manor which sheltered three generations of Cahills including Gideon and Olivia Cahill, and their children, Luke, Jane, Katherine, Thomas, and Madeleine. Gideon's alchemy lab and the Vesper Manor was located on Cahill Island.
It has a flat beach and a graveyard containing the graves of the original Cahills, but its defining feature are its cliffs, thousands of feet high, atop which lies the elevator entrance to the Madrigal stronghold known as the Gauntlet. Within this stronghold is a museum and a maze. Thirty-nine doors must be unlocked by a series of questions in order for them to enter Gideon's laboratory. When Amy, Dan, Alistair, Natalie, Ian, Jonah, Sinead and Hamilton enter the Gauntlet, all of the doors have been destroyed (because of Isabel using explosives to enter the place) except for the last door. At the end of the Gauntlet there are the remains of Gideon Cahill's lab and many rooms where the Madrigals tried to reconcile the Cahill family. All of the attempts failed. The last and most recently built room is where the final Clue, the Serum Formula, can be found. Damien Vesper also lived on the Island while the mainland was being infected by the plague known as the Black Death. Because the island could not be seen from the mainland and the Madrigals bribed mapmakers to keep it a secret, nobody knew about it for years until the end of the Clue Hunt, which took the contestants there.