The Drunken Boar is an NPC you can meet in Chapter 2 of whose quest leads to a secret area. Like other questlines, you'll have to retrieve items, fight bosses, and listen to this character's dialogue to make progress.
Following the Drunken Boar's quest to completion will lead to rare rewards, including a way to find a hidden ending to the game itself.
Before starting this quest, it is recommended that you make at least enough progress in Chapter 2 to defeat the First Prince of Flowing Sands boss.
This optional foe can be faced in the Valley of Despair after you take down his father and brother at other points in the area.
You should get an item called the Arhat Gold Piece for beating the First Prince, which will be important later for the Drunken Boar.