This book provides a lot of detail about the world of Ravenloft over what was previously established in adventure modules for 1e. It adds several rules over the standard 2e PHB, such as fear/horror checks, alterations to class abilities such as turn undead, and several spells have been altered or reduced in effect. A few new spells have been added as well as some magic items. The fear/horror checks can be a fun mechanic, although they suffer from the clunkiness of a lot of AD&D 2e content - you may be better served by borrowing a mechanic from a game designed for horror from the get-go.

Like a lot of the published material for table top RPGs, this suffers from too much world-building and not enough adventure design. There are about two pages of very vague adventure ideas, compared to 68 pages about the geopolitical regions, their leaders, and individual backgrounds and family trees. That also does not include the several pages dedicated to scans of the fold-out maps and isometric drawings of the castles. That stuff can be fun to read, but it's not terribly useful for a DM. I personally find world building to be the easiest aspect of DM'ing, and it certainly is the laziest thing to write for a supplement since the authors didn't have to deal with any pesky play-testing or anything of the sort when writing those things.

That said, Chapter XV: Techniques of Terror contains several pages of advice for DMs to run horror games - this chapter alone is worth the $5 for the digital copy, as it provides a lot of advice for the DM on how to create the atmosphere of a horror game for your players. Some of it is a bit dated and not very practical, but some of it is really good.