The cave was built by the Matoran of Gruissiono when Bruhodag told Trasder to do that. He said that there should be a cave where the Matoran could hide while the doom of Gruissiono came. Trasder ordered the building of the cave, therefore. When the cave was done, Bruhodag came to Gruissiono and the Matoran fled to the cave. Bruhodag then fought Trasder destroying him and the city. The Matoran remained in the cave and thought that Bruhodag had trapped them there... even though the door to the prison wasn't locked!