This locked door is found in the Temple of Miraak Sanctum in the same chamber where you learn the Dragon Aspect shout. The key to this door drops from the Gatekeeper.
Skyrim Dragonborn: Temple of Miraak Locked Iron Door
From Orcz
This locked door is found in the Temple of Miraak Sanctum in the same chamber where you learn the Dragon Aspect shout. The key to this door drops from the Gatekeeper.