You get this achievement for allowing
After the "Capture the Castle" main quest, you start "The Night of Longfangs" quest. This quest has a choice in it. You can choose to find Syanna or you can choose to look for the Unseen Elder. In order to get this achievement you have to choose the former.
When searching for Syanna, you'll end up in the Fablesphere. The main quest within the Fablesphere ("Beyond Hill and Dale") has a number of parts to it that can be done in any order. One part in particular is to talk to the Flint Girl. This is the key task to avoid for getting this achievement. The Flint Girl has a ribbon which used to belong to Syanna. In order to get the achievement, you need to
NOT get the ribbon. Simply don't even talk to her at all, or if you do, do not buy or try to win the ribbon.
Proceed with the rest of the quest (skipping the ribbon) and you'll leave the Fablesphere, set up a series of cut-scenes with minimal interaction, and at the end of that section, you'll find yourself
and the achievement will unlock.
While this is basically the end of the DLC, note that there is quite a bit of extended conversations and minor tasks to flesh out the full ending after the achievement unlocks.
Note that you cannot get this achievement and
at the same time. If you want to get both with minimal replay, choose Syanna's path and make a manual save near the Flint Girl. Get or avoid the ribbon, finish the game, then reload and make the other choice with the ribbon and go from there.