The Lightheart is a magical sword. It is made out of an iron rod for the handle and quartz for the blade. Its counterpart is the Darkblood. Despite the creation of the Metal Crucifix, Villainbrine had to build this because he had to give the crucifix to Steve, therefore forcing Villainbrine to build another sword with pure heroism to battle his own brother when Steve has died, which is unlikely, or when the time comes, and Villainbrine must defeat his own brother, he must ask Steve on which side he would choose; Antagonist or Hero. When he chooses to be an Antagonist, Steve, and Villainbrine with Herobrine will attempt to defeat Steve, and when he chooses to be a Hero, Steve and Villainbrine will be teamed up, and Villainbrine will have no worries about his own weapon.
The Lightheart is one of the weapons essential to saving Minecraftia, with the Metal Crucifix which's counterpart is the Evil Crucifix. The Lightheart is mythologically significant in defeating Herobrine, while the Darkblood is essential into defeating Villainbrine, though both myths were only for the present and no one knows what the Lightheart and Darkblood's true purpose is. Probably everything mentioned above except the mythology that the Lightheart and Darkblood were made to defeat the opposed side of their owners, in this case, Villainbrine of the Lightheart and Herobrine of the Darkblood.