From the front of the book:
Far below the earth, disaster has struck, bloody and red. The bastion home of peaceful gnomish miners has fallen prey to a shadowy and mysterious evil. Can a certain band of heroes drive out this menace from the gloom before it rises up to claim the unsuspecting world above?
This module provides the adventurers with three distinct settings of adventure: a large cavern within which the three-level tower rests; two levels of mines below the tower; and the sinister lair of forces allied with the tower invaders. Tower of Blood also features unique NPC foes, seven new magic items, and five new monsters.