After Nephi had quoted the Isaiah passages, he started going through the four main elements in his prophetic world-view. In verse 12, Nephi talked about how Christ would come and how Jesus would manifest himself to the people in Jerusalem but they would reject and crucify him. Nephi included new information here that had not been mentioned before in the Nephite record, specifically he prophesied that after Jesus was laid in a sepulcher for the space of three days, he would rise from the dead. In 1 Nephi 19, Zenos had been quoted as saying there would be three days of darkness, but it did not mention what was going on during those three days of darkness. Here we learn that there would be a three-day period in which Jesus would be in the tomb.
Book of Mormon Central, "What Caused the Darkness and Destruction in the 34th Year? (3 Nephi 8:20)," KnoWhy 197 (September 28, 2016).