The Heartseed is the seed that the World Tree grew from. It was torn apart by Princess Lavender, for which she used to grow Briarheart Palace. It is a treasured item found in Memorial Caverns that holds great historical value.
When Queen Ayani passed, she left behind the Heartseed. With the royal family nurturing it, the Heartseed grew into the mighty World Tree, the center of the pixies' world.