Emperor Jewel is a relic piece obtained by defeating Emperor, the 2nd Boss in the Imperial Tomb. Alternatively, you can purchase it from the Adventure Store for 650 Smith Tokens once you have 3-Star Mastery in the Imperial Tomb.
The other pieces you need to create National Treasure are the Rank-A Gold Sword, The Gold Cannon, and the Rank-A Diamond Sword, Eternity. As you can see in the picture below, the below fusion is ready as indicated by it giving you a checkmark to proceed.
Successfully fusing the 2 components together over 36 hours will result in your sword.