Ballad Heroes is one of 16 free DLCs for The Witcher 3: Wild Hunt that brings a new appearance to several gwent cards. It was released on 12 June 2015 and you can switch to the new look in the Game Menu.
Official statement
Make your Gwent Deck legendary! The "Ballad Heroes" neutral Gwent card set will provide alternative cards capturing characters from Dandelion's ballads in all their glory.
Gallery
All neutral card normal and ballad heroes