A fierce deity in Vedic times.
When the Dravids immigrated to India and brought Shiva with them, he had to be integrated into the pantheon. This is how Rudra became Shiva. Shiva means ‚benevolent‘. For his fierce aspect, Rudra remained and Shiva became the destructive (die and become) aspect in the trinity Brahma, Vishnu, Shiva.
A famous hymn is Shri Rudram
http://www.skandagurunatha.org/works/stothras/sri-rudram.pdf