Undvik boots are heavy armor boots and part of Skellige Armor set free DLC for The Witcher 3: Wild Hunt on June 3, 2015. They can be bought from Armorer in Kaer Trolde citadel and cannot be dismantled. In the various elements of the armor, favored by Geralt during his treks across the unforgiving tundra of Skellige, strong influence of Undvik mastercraft can be seen.
Whether due to a bug or intentionally, the upgraded New Game + version can be purchased in a regular game as well. At level 41, it will replace the level 16 version in the armorer's inventory. Similarly, it cannot be purchased before reaching level 41 even in a New Game +.
See also
Gallery
Final concept by Marek Madej
Various sketches from development.
Full set from every side