The 'Mystery Game - 1 Pack' look to be in Green Man Gaming's Winter Sale
. Use the voucher WINTER10 to get a free mystery game and 10% off the £0.49 price. GMG say:
"Are you feeling lucky? Get a fantastic title in our new Mystery Bundle. Buy now to uncover the secret keys within.
NB. Do be aware that this Bundle may contain Steam, Origin, GOG or Uplay keys. Check your key delivery email for further information. Game keys are allocated randomly and customers may receive the same game twice, or a game they already own."