In Dreadspire, the Demiplane of Blood: Part of the raid event "Roley and Adrianna" (see this quest entry for details).
In Miragul's Nightmare: Part of the partisan task "The Sword of Shadows" (see this quest entry for details).
In the Heart of Fear: This chest spawns upon completion of the Heart of Fear mini raid event "Ulrich the Ageless" (see this quest entry for details).