Ronnie James Dio was a legendary heavy metal singer, and one of the forefathers of modern metal. He spent time in bands like Rainbow, Elf, Black Sabbath and Heaven and Hell, but was probably best known for his solo band, simply called Dio. He's also often credited with making the "devil horns" hand gesture popular among metal fans. Some of his most epic songs were Rainbow in the Dark, Man on the Silver Mountain (Rainbow) and The Last in Line, but truly the impact of his music is too vast to fully examine here.
Dio passed away back in 2010 and he left behind some incredible music, but most importantly he has influenced musicians in just about every genre of metal, especially power metal. RIP Ronnie. Metal will never forget you.