What is "shadowmoor"

Shadowmoor is a Magic: The Gathering block consisting of the expansion sets Shadowmoor (May 2, 2008) and Eventide (July 25, 2008). It was originally planned as part of the Lorwyn block, but partway into the design of Lorwyn, that block was changed to a two-set block and Shadowmoor was released as the first set of a second two-set block. The two blocks were linked through synergy and rotated through the official tournament formats as a single, four-expansion unit.