Redeeming a Path of Exile: Sentinel - Mystery Box key is really simple, just follow these steps:

1. Enter your key at the Path of Exile shop.
2. Redeem the code. 
3. Go to the "Consumables" section of your microtransaction stash and collect your Mystery Box. 

The key is not working

If you have activated the key according to the guide but the code appears not to work, please check what type of error message you receive and depending on it follow the appropriate instructions:

• My key is Duplicated / Already used
• My key is Invalid
• My key is region locked / can't be activated in my country
• My key activated a different product
• My key did not activate all of the content