Skandar and the Skeleton Curse (Skandar #4) by A.F. Steadman - Used

As Skandar and his friends begin their fourth year at the Eyrie, the Island’s unicorns are struck by a terrible curse that threatens to change everything. Between a Commodore determined to eliminate the spirit element for good and a sister hellbent on revenge, nowhere is safe for Skandar.

As more unicorns are affected by the curse, the clock is ticking for Skandar and his quartet, who find themselves racing for their lives. Can they stop the curse in its tracks before the Island is lost forever?