This achievement is unlocked for completing the Dark Brotherhood Questline.
The following is the list of quests if you need to check how near/far away you are from earning the achievement
Dark Brotherhood Questline
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- Innocence Lost
- (10G) With Friends Like These
- Sanctuary
- Contract: Kill Narfi
- Contract: Kill Ennodius Papius
- Contract: Kill Beitild
- Mourning Never Comes
- Whispers in the Dark
- Contract: Kill Lurbuk
- Contract: Kill Hern
- The Silence Has Been Broken
- (10G) Bound Until Death
- Breaching Security
- The Cure for Madness
- Recipe for Disaster
- To Kill an Emperor
- Death Incarnate
- (30G) Hail Sithis!