The Year of Horror was a conflict between the Klingon Empire, the United Federation of Planets and the Orion Colonies during the Four Years War.
On reference stardate 1/98 (c. the year 2256), as the Klingons retreated before the advancing Starfleet forces, they launched a scorched earth policy on the Orion worlds they'd once occupied, in what became known as the Year of Horror. They damaged industries and spoiled local ecologies, executed leaders, massacred people, and committed other atrocities. The civilian populations were left demoralized and starving. (FASA RPG - The Orions module: Book of Deep Knowledge)
However, the Klingons left the core Orion worlds of the Rigel system untouched, apparently in an attempt to make them and the Botchok Planetary Congress (BPC) appear to be Klingon allies. Thus there was a wave of anti-Klingon riots and mob violence in the Night of Empty Hands, which achieved much the same outcome. (FASA RPG - The Orions modules: Book of Common Knowledge, Book of Deep Knowledge)
The problems continued for the Orions as Federation forces occupied their worlds in turn. The Federation conducted war crimes trials against surviving Orion leaders, accusing them of aiding the Klingon war effort. They also took control of civil governments and industries and placed restrictions on commerce that Orions found intolerable, supporting the new rules with a military presence that the Orions could not resist. This prompted the flight of Arthas Liktor the Bold of Hartha in stardate 1/9805 in an unsuccessful attempt to warn of the tightening Federation grasp.
By stardate 1/9806, wild rumors were spreading through the Colonies, telling of battles past the Klingon front lines, the destruction of Klingon fleets, secret Klingon super-weapons, and the looting of peaceful Orion worlds, among other things. Refugees fled and defense committees were established everywhere.
The last great problem of the Year of Horror came when Starfleet's Eighth Fleet arrived in the Rigel system and sealed it off, then landed soldiers on the homeworld of Rigel VIII (Botchok) to capture the spaceport and communications centers. The Federation's Rigel Demilitarized Zone Commission then forced strict borders on the Orion Neutrality Area and the Orion Emigration Act on the Colonies, amidst riots in the BPC chambers. (FASA RPG - The Orions module: Book of Deep Knowledge)