You unlock this by completing the optional boss fight with the Executioners Chariot in Huntsman's Copse. After you have killed him you need to look for the guy stood by the bonfire, he will ask you a series of questions which you have to agree with.
To be able to unlock this trophy you need to have at least on token of spite in your possession. These are obtained by invading and killing other players. There is one in a chest in the game if you don't want to invade people. You need to make your way to the very bottom of the pit in the centre of Majula and just before you reach the Black Gulch area there is a chest containing a token of spite.
Here's a video of the covenant's location.