Ralsei explains Dark Worlds with the exact line: "Your eyes can't see the truth anymore."
The SOUL dispels falsehood and reveals truth. It does the same to the titan spawn and the titans; it will do the same to the Roaring Knight and reveal the truth about Dess.
It will reveal the truth about what happened all those years ago; it will reveal the truth behind the nature of DELTARUNE. For better or for worse, it will make characters confront that truth, which will either break them or make them stronger.