Mordenkainen's Lost Notebook
5-level Transmutation
While heart of fire is in effect, you gain a 10-foot bonus to your movement speed. In addition, you gain resistance to fire damage. Furthermore, while heart of fire is active, you can use your reaction to cast the fire shield spell on yourself. Once the fire shield effect has been used, heart of fire ends. If heart of fire is active on you at the same time as heart of air, heart of earth, and heart of water, you become immune to extra damage from critical hits and sneak attacks.