The Shadow in the Force was an unprecedented cataclysm of massive proportions caused by the destruction of the Force-world Mortis. Hundreds of the planets in the known galaxy were rendered dead worlds and asteroid clusters, and hundreds, if not thousands of unique species were wiped out as a result. Afterward, the area became known as the Cold Sector.
A result of the Shadow Crisis was the decrement of Force abilities in force-sensitives, and this further caused the Jedi to seclude themselves on Illum in the following years.