Unhallowed Halls is a rather unusual d20 adventure. Unusual in that it's, to my knowledge, the only d20 adventure set at a magic University. Not the Hogwarts of Harry Potter, it seems a bit more something out of Animal House (or at least far more interesting than the college I went to.) The cover says it's for characters of levels 6 to 8, but the module itself says 4 to 6. It's softcover, 80 pages, and priced at a $16.95. The margins are also quite small, so in terms of text, it's a pretty good deal.
The set up is also a bit odd, but not unique - the characters are invited to a ball (like in a party. If you have players that are AC/DC fans, beware). At least one other d20 module starts off like this, one of the Freeport ones. But in that case, the PCs were invited to the ball because they were heroes of the city, at least temporarily, because of the previous adventures. In this case, it's somewhat tricky to explain why the PCs have been suddenly invited to a ball at a college that never existed in your game world until you bought this book, though several possibilities are suggested.