This is a DLC for a F2P game.
It is the usual procedure.
Click on the link, scroll down to the Giveaway field, put in your e-mail address, press redeem, follow the instructions, and wait for a bit until you get an e-mail with the Steam key. Then redeem to your Steam account.
You will get:
Slimmy + Silver Magnetic Collar - pet to collect your loot for you
Consumables: 20 Maiden's tickets to fast travel between saved locations; 20 Elmar's Balm to instantly heal your character; 20 Melios' Tincture to instantly replenish your characters' Energy or Mana; 20 Ambrosia to heal and replenish Energy or Mana over little time; Yellow Box which can contain even most valuable loot!
Have fun.