Flaming Heart is a relic piece obtained by defeating Ifrit, the 2nd Boss in at Flaming Volcano. Alternatively, you can purchase it from the Adventure Store for 450 Smith Tokens once you have 4-Star Mastery in the Flaming Volcano, or the equivalent Heroine Quest.
The other pieces you need to create Ifrit's Fury are the 3 Rank-A Fire Stone Swords; Skyfire, Phoenix, and Hellfire. As you can see in the below picture, it will tell you how many correct swords you have. The below picture has none with a gold border, so it still needs all 3 Rank-A swords. If you hover your mouse over each picture, it will tell you what Rank-B sword each one needs.
Successfully fusing the 4 components together for 36 hours will result in your sword.