The Lysian destroyer was a small warship of the Lysian Alliance, far weaker than the Enterprise-D, one of which the amnesiac Starfleet crew destroyed in 2368 while being manipulated by a Satarran.
Lysian destroyers appear in the Next Generation episode Conundrum. An alien scan wipes the crew's memories, and altered records tell them that the Federation is at war with the Lysians and that the Enterprise must cross their border and destroy their central command. An unfamiliar first officer, MacDuff, urges them on.
A destroyer intercepts the Enterprise and attacks. Under what he believes to be his orders, Picard has it destroyed, and is disturbed at how little effort it takes: the ship's weapons could not have harmed him.
The same imbalance at the central command, defended by sentry pods no better armed, convinces Picard that something is wrong. He refuses to fire, and MacDuff is exposed as a Satarran whose people wanted to use Starfleet's power to win their own war.