There have always been a lot of secrets in the Souls series. Given that Dark Souls 3 is the final DLC for the franchise, it was only proper to host a hidden boss.
Before we get started, note that below you will find general location information that may be interpreted as spoilers. With that said, let's get started.
To begin, head to the Ringed Inner Wall bonfire, a location just a few minutes into the DLC. From the bonfire, head across the main path until you find a staircase to your right. The NPC here is who you want to speak to. Accept the quest he offers.
The quest asks you to defeat a dragon. The dragon in question is located near the Shared Grave bonfire much further into the DLC. From the bonfire, go up the stairs and you will find him.
Upon defeating the dragon, head to the Church bonfire. In front of it is an area with a large number of statues. Nearby is an elevator with a hidden alcove. You want to jump into this alcove before finding the statue at the end of the nearby hallway. This statue is your gateway to the hidden boss. Simply hit the wall with your weapon to reveal a secret passage.