The Gatekeeper Trilogy Series by Christopher Golden, Nancy Holder

Sunnydale is being besieged by dark forces. But even with Buffy providing her unique style of damage control, it's more than one Slayer can handle - especially since the abominations are coming from a centuries-old portal through time and space.https://openlibrary.org/works/OL14963...

Passport to HellBuffy, Oz, and Angel are Europe-bound, only they're not flying any airlines. They're traveling limbo's "ghost roads" in search of Jacques Regnier. Jacques is the sole heir of the dying Gatekeeper whose Boston mansion is the supernatural barrier restraining thousands of the world's monsters. The evil Sons of Entropy will do anything to destroy the gate - even if it means trading the power-laden Spear of Longinus to the wicked vampires holding Jacques... morePassport to HellBuffy, Oz, and Angel are Europe-bound, only they're not flying any airlines. They're traveling limbo's "ghost roads" in search of Jacques Regnier. Jacques is the sole heir of the dying Gatekeeper whose Boston mansion is the supernatural barrier restraining thousands of the world's monsters. The evil Sons of Entropy will do anything to destroy the gate - even if it means trading the power-laden Spear of Longinus to the wicked vampires holding Jacques.Back home, the ghost ship "Flying Dutchman" has set sail for Sunnydale, determined to shanghai new crewmen - dead or alive. For Willow, Xander, Cordelia, and Giles, it's an ocean of trouble, especially when the monstrous Kraken reemerges with a vengeance.But everyone's assistance will be needed once Buffy locates Jacques, and uncovers the shocking plans the Sons of Entropy have already placed in motion - a plan that, if successful, will destroy the world and create a horrible new realm ruled by monsters. less

Led by fanatical Il Maestro, the Sons of Entropy are assaulting the supernatural Boston mansion that holds back the realm of monsters. With the Gatekeeper weakening, Buffy sends Willow and Cordelia to escort Xander along the Ghost Roads to the Gatehouse.