The whiskies in Cadenhead's Enigma series are just that, unusual, hard to quantify, perhaps even slightly contradictory. They can be single casks, vattings of multiple casks, blends, blended malts or whatever else they find that doesn't fit into their existing bottling lineup. As long as it meets their essential criteria - that they're good 

Distilled in 2000 and aged for 23 years in a single Bourbon hogshead, before being bottled at natural cask strength of 51.8%ABV. No chill filtering or artificial colouring. 

Only 318 Bottles have been produced.