From Li He's (
李贺) Mid-Tang poem 《
雁门太守行》 (Ballad of the Governor of Yanmen):
黑云压城城欲摧,
甲光向日金鳞开。 — 'Black clouds press on the city, the walls about to crumble; armor glinting in the sun opens like golden scales.' A poem about a desperate border siege, now the go-to line for any oppressive political or natural crisis.