To get this achievement you have to finish the main plotline of Greymoor expansion, as well as complete the following questline ending with "Second Chances":
- The Despot of Markarth
- Blood of the Reach
- The Study of Souls
- The Awakening Darkness
- The Dark Heart
- A Feast of Souls
- Kingdom of Ash
- End of Etenity
- Second Chances
Please note that the first six quests are doable without the Greymoor plot finished, but you won't be able to start the latter three until you are done with the western Skyrim main quests.