From the back of the book:
The Black Monastery is a classic style adventure module for parties of five to seven characters of seventh to tenth level, with the average player character being ninth level. The adventure includes maps and descriptions for the monastery's main level, two towers, and three dungeon levels. There are two separate dungeons, one of which has two levels. There are enough things to do, choices to make, and monsters to fight to occupy a gaming group for a dozen gaming sessions or more.
The module is written in the "sandbox" style that allows players to choose which way to go and which challenges to face. There are no railroad situations and it is better for the party if they avoid some of the things they find or creatures they meet. There is no climactic battle, but a party of very foolish adventurers might not survive long depending on their choices.
They might not even survive the first encounter.