Item from deaths-end by
A sentient four-dimensional ship that calls itself 'Tomb,' encountered in a collapsing 4D bubble — and the universe's clearest first-person testimony of dimensional war.
The Ring is the most cosmologically important entity humanity ever speaks to. It is a sentient 4D vessel, abandoned to a collapsing fragment of higher-dimensional space; it is functionally dead inside, with no living crew, but maintains enough cognition to communicate. It calls itself 'Tomb.' It explains to the Blue Space and Gravity crews that 4D space was largely flattened in some prehistoric dark-forest war, and that the dark forest exists between civilizations of the same dimension but not across dimensional levels. Its testimony is the seed of humanity's understanding of the dimensional weapon waiting for them. It dies when its bubble collapses again.
From a 3D viewpoint, a translucent torus of impossible internal complexity — every part of it visible at once, including its long-dead crew. Inside the 4D bubble, it is incomparably larger than it appears from outside, full of organs and chambers that occupy directions the human eye cannot follow.