Prosecutor Nicola Gratteri has spent the past three decades battling Italy’s most powerful mob group, the ’Ndrangheta, renouncing any semblance of a normal life as he seeks to break its grip on his native Calabria.

The toe of Italy’s boot, Calabria is a rugged backwater from where the ’Ndrangheta operates its multibillion-dollar crime syndicate, controlling chunks of Europe’s cocaine trade and laundering vast sums through a network of front businesses...