Terrible news - Dio had been suffering stomach cancer for some time, but last I'd read it was said to be in remission.
There's another of the gods of rock 'n' roll lost to us, though at least he made it to a, well, sort of reasonable age, 67. Of course, that's still far too young. I was privileged to see him perform live on two occasions, once back in the '80s, when he was touring with his own band, and again very recently when he was fronting the reunited Dio-era version of Black Sabbath (officially known as Heaven and Hell). Even in his sixties, Ronnie James Dio was an exciting performer and still the quintessential heavy metal vocalist. I'm so glad I had the opportunities to attend his performances.
My sympathies to his loved ones.