The Seventh Sacrament

The Seventh Sacrament is the fifth in the Nic Costa series, David Hewson's detective novels of love and death in Italy.
There's an entire underground city down there . . . houses and temples, entire streets. I talked to a couple of the cavers Leo called in. They hero-worshipped Giorgio. The man had been to places the rest of them could only dream about.
Giorgio Bramante, a Roman archaeology professor, was master of the hidden world beneath the earth – until the day he lost his young son, Alessio, to a group of students intent on re-creating a centuries-old ritual to a long-banished god. His rage knew no bounds and, in a frenzy, he beat one of the students to death.
Released from prison fourteen years later, Giorgio is bent upon a terrifying revenge on all those he blames for the loss of his son. Inspector Leo Falcone, a member of the original investigating team, is one of his targets.
And Nic Costa, watching Falcone move relentlessly into the man's merciless grip, realizes the answer must lie in solving a cold case that, like the forgotten Alessio Bramante, has long been regarded as dead and buried for good.