Story related, cannot be missed.
This achievement is obtained after you first arrive in the new island of Solstheim, during the very first quest to start the DLC. To start the DLC, you must be inside one of the major cities, where you will be attacked by cultists. You will recognize them as they will be wearing robes which are only just introduced in this DLC. After dispatching your attackers, search both of their bodies to find a note and read it. This will then activate a quest for you to go and speak with a ships captain, who can be found on his ship just outside Windhelm. After a small conversation, he will eventually, albeit, reluctantly agree to take you to Solstheim, thus starting the DLC.
Also, it seems you have to have completed the mission, 'The Way of the Voice' in the main Skyrim game and received the quest 'The Horn of Jurgen Windcaller' for the attack to trigger.