After defeating the Jailer in the Sepulcher of the First Ones raid, there is a new skybox in Icecrown Citadel. Story spoilers.
The new skybox, using the dark blue pre-Shadowlands appearance, reflects how the veil between Icecrown and Shadowlands has closed once more, as we saw in the Jailer Finale Cinematic upon his defeat: Throughout all of Shadowlands, the sky above Icecrown Citadel was shattered in-game, reflecting the events of the Shadowlands Intro Cinematic. Here, Sylvanas ripped the Helm of Domination in two, sundering the veil between Icecrown and the Shadowlands.
This iconic sky also served as the login screen for the Shadowlands expansion. This isn't the first time we've seen skyboxes reflect story events, as back in Legion, we saw the planet Argus after Kil'jaeden's defeat, replaced with a faint red star after clearing Antorus.
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