Found among a bunch of unredeemed keys from Humble Indie Bundle 7, bundled as "Dungeon Defenders + All DLC", which SHOULD match the selected game, but I am unable to confirm. At least the 2012 summer-winter range matches, the bundle was bought on the 20th of December, 2012. Also see: https://indiegamebundle.fandom.com/wiki/Dungeon_Defenders
Could not find the game on the revoked keys list, but if it does not work, please allow me to delete.