Dewar's Signature Blended Scotch is the brand's most premium expression, made from a blend of "the rarest of rare whiskies from across Scotland," according to the marketing materials. It then undergoes a secondary aging in oak casks to marry the flavors together.
Dewar's was founded by John Dewar, Sr. in 1846, later taken over by his sons, and eventually worked its through the corporate maze to become part of Bacardi. The brand was popularized in two fairly unconventional ways: Tommy Dewar, one of John's sons, was a successful travel journalist who used his platform to promote the family business, and Andrew Carnegie sort-of-famously sent a cask of Dewar's to the White House to celebrate Benjamin Harrison's inauguration in 1889. It has since become one of the best-selling blended scotch whiskies in the world.