Vader floats inside his bacta tank, soaking up nutrients while contemplating the Rebels’ demise from the security of his Barad-dûr-like fortress of evil.
For us, it was really figuring out that Vader built his castle on an existing structure, which was this ancient lava dam.
Why does it look the way it does?
And it became a very spiritual place, and it was all centered around his bacta tank. And that’s where the form language all built around there.