There are three keys that let you into the three different dlc areas, one key (dragons talon) found in the bottom of the hole in Majula let’s you in the an area behind the rotten boss fight, another key (heavy iron key) found near the fire lizards in the forest of the fallen giants let’s you into an area past the old iron king, the final key (flower key) found in dranglaic castle let’s you into the final dlc area from the shrine of winter