The journal holds Corona's secrets that were made in Der Sonne's time, as well as acts as a map to the underground tunnel system, that were placed under the kingdom during its time of war with Saporia. It also holds the king's feelings of love toward the leader of Saporia, General Shampanier. When she saw his confession of love in the journal, during their battle, Shampanier realized that she felt the same way toward the King of Corona. The journal plays an important part of a Coronaian holiday that celebrates the ending of the war and Der Sonne and Shampanier's marriage merging the two kingdoms together, which would forever be known as the "Day of Hearts". Every year since then, couples would write their names in the journal to proclaim their love for each other and for it to be displayed to the public.
Role in the series
Because of the secrets that it holds, the Separatists of Saporia had tried to get their hands on it. One of the separatist members, Andrew, had used the Day of Hearts holiday as an opportunity to steal it by building up a fake relationship with Cassandra, who was onto him from the start. Despite getting his hands on it and was now on his way back to Saporia so they could use the kingdom's secrets to plan an attack on Corona, Cassandra, Rapunzel, and Eugene stopped him and brought the journal back to the castle.
When Rapunzel and Varian agreed to steal the sun-drop flower, in "The Alchemist Returns", they used the map within the journal to help them find their way around the Der Sonne's tunnels so they could break into the guarded vault, that held the flower, from beneath. As the two were stealing the journal for their task, Rapunzel was hesitant about what they were doing, as she knew that it would lead to her committing treason toward her own family and kingdom.
During the events of "The King and Queen of Hearts", it's the annual Day of Hearts celebration and Rapunzel attempts to help her parents regain their memories by showing them Herz De Sonne's journal since they proclaimed their love by signing their names in the book, but to no avail. Eugene also expresses his wish for him and Rapunzel to sign the book together, since they are the only ones yet to proclaim their love in the journal. Rapunzel assures Eugene they will sign it, but wants to focus on helping her parents. The journal is later stolen by King Trevor for his plot of making Arianna his queen. Trevor presents the journal to Arianna, asking her to sign it with him and rips out the page where she and Frederic previously wrote their names in the past.
By the end of the episode, when both parties returned to shore, Trevor, having failed to win Arianna's heart, gives the journal back to Rapunzel. Unfortunately, it had gotten wet in their battle with the sea serpent and all of its long held contents from Herz Der Sonne's time to there own were washed away. Rapunzel is saddened by the journal's destruction since it contained precious memories for her parents, but Frederic and Arianna comfort Rapunzel, reassuring they will make new memories. Inspired, Rapunzel creates a new Book of Hearts and she and Eugene are the first to sign their names, finally and officially proclaiming their love for each other.
- In the opening of "Under Raps", the journal was featured as a popup book as it told the origin story of the Day of Hearts and how love ended the war between Corona and Saporia.
- King Herz Der Sonne's journal is also known as the Book of Hearts.