You get this achievement for completing the "Turn and Face the Strange" quest (to be precise, the achievement will unlock very late in the quest).
You'll get this quest moderately early in the DLC. After arriving in Touissant, examining the latest murder scene and body, meeting with the Duchess (and the events that precede the meeting), and dealing with the scavenger hunt , you'll end up in a cutscene with a nice chase, followed by a battle, and then a conversation with an old friend of Geralt's from the books who hasn't been in any of the games yet. After the conversation, when you leave the area, you'll follow a short path by some buildings where a boy messenger will be waiting to give you a letter. The quest starts when you get the letter.
In brief (non-spoilery, with just an outline of the steps involved), the quest requires you to
1. Search the local cemetery for some clues.
2. Look at a map which came with the letter.
3. Travel to the location marked and search for an underwater entrance.
4. Work your way through a cave system to a laboratory, including some minor puzzle solving.
5. Fight a battle or two.
6. Perform a simple task in the lab.
After this last part the achievement will unlock and all that remains in the quest is to leave the area.