A Dark Matter by Peter Straub

Mass market paperback format.  Recommended!

An electric, chilling, and unpredictable novel from the master of modern horror.

On a Midwestern campus in the 1960s, a charismatic guru and his young acolytes perform a secret ritual in a local meadow. What happens is a mystery—all that remains is a gruesomely dismembered body and the shattered souls of all who were present.

Forty years later, one man seeks to learn about that horrifying night, and to do so he’ll have to force those involved to examine the unspeakable events that have haunted them ever since.