After you have defeated both the Ice Sentinel and The Malevolent Worm, forest hearts will spawn randomly where grass and trees can spawn / in the forest biome. Just a warning, that boss significantly harder to kill than both of the previous bosses. If you want a semi-easy win against it, leech in the skill tree gives the player a lot of sustain or just a bow from a large distance as it can't move. I am working on a spawning system where you don't have to find the heart to spawn the bosses and you can craft the spawner after you collect materials for it instead. Good luck if you haven't killed it already!