Item from deaths-end by
A white slip of paper the size of a credit card, fired by Singer at the Solar System — a dark-forest weapon that collapses 3D space into 2D and never stops spreading.
The dual-vector foil — er xiàng buó, 'slip of paper' — is the strategic dark-forest weapon of choice for civilizations that have given up on collateral damage. It is more destructive than the photoid: where a photoid kills a star and sterilizes the system, the foil collapses the very dimensionality of the region around it. Within roughly 30-50 AU of detonation only objects exceeding lightspeed can escape the collapse. Worse, the collapse front never stops spreading; a single foil released anywhere will eventually flatten the entire universe, including the civilization that fired it. Using one is therefore an admission that one's own civilization is already fleeing into pocket universes or has accepted its own end.
Visually, a thin white rectangle, no bigger than a credit card, glowing very faintly. It floats in space without thrust, indifferent to gravity. As it activates, a translucent membrane unfolds from it and sweeps outward; matter caught in the membrane is drawn smoothly into two dimensions, retaining all its surface detail.