Horizon

The horizon is the annular region around the TwinWorld axis where the Adamant and the Outer Fire converge. What with boiling adamant, searing flame, and intense gravitational torsion, the horizon is not a place to build your summer home. Try the Great Tree instead.

However, if you want to craft artifacts from super ores, or even adamant, the horizon is the place to be. The outer fire makes all substances more pliable and there is an ample supply of energy in all forms. Just don't get caught in a ripple, or you'll be trapped, consigned to who knows what fate. None have ever returned from over the horizon, so be wary!

Symbolism

To me, the horizon represents death and the inevitable undoing of all works and effort. It is entropy. The narrowing of options until none are left. The ultimate dissipation of all physical triumphs into unknowable oblivion.