For The King II

While down in the abandoned sewer during Resistance campaign there was a market place which had a SKELLY KEY to purchase for 100 coins. In the next area the Bandit King will give you a quest to clear out Boogey's Crypt. While in the dungeon you'll come to a room with a locked door in the background. After the battle select STRAIGHT and you'll head into the secret room!

There's 2 exits out of the first area, the cellar didn't have the Skelly Key option to purchase for me.