The Valley of the Jedi, or the Valley of the Souls, was the site of the Seventh Battle of Ruusan, the last battle of the New Sith Wars and a thousand years before the Battle of Yavin.
There, Lord Kaan's thought bomb trapped the souls of over one-hundred Jedi and Sith until they were finally freed by Kyle Katarn.
Kyle Katarn journeyed to Ruusan in 5ABY, pursuing the Sith Lord Jerec, who sought to harness the power of the fallen Jedi contained within the Valley.
The Valley of the Jedi then became a concentrated area of Force energy called a Force nexus, and is widely believed to be the most powerful Force nexus in the entire galaxy.