If the player managed to collect the five hidden sub-weapons, a secret ending is unlocked which is played after the normal ending. Pictured is Sonia holding a baby while the following text scroll is displayed:
- After some time, the young girl of our legend became a mother, whose child would carry on the fate and tragedy of the Belmont family, and the bloodline of dark ways. A child burdened with a cursed fate. And let this child, once content in the love of its mother, will also rise to fight courageously against the Prince of Darkness, who will return once again. This child who, one day will be praised by all the people as a hero, but that legend must be told another time.
The developers of Castlevania Legends have never confirmed the identity of Sonia's child. However, because the game's has been advertised as the origin story for the Castlevania series, and the fact that the game's ending alludes to a future confrontation between the child and Count Dracula, there's a strong possibility the child is supposed to be an already established character. Possibilities include the following:
The most prevailent theory is that the child's identity is supposed to be Trevor Belmont from Castlevania III: Dracula's Curse.This theory assumes Castlevania Legends follows the chronology established by Koji Igarashi during the development of Castlevania: Symphony of the Night. In this chronology, Dracula's Curse is placed in the year 1476. Because supplementary material places Legends in the 1450, it's assumed the events of Dracula's Curse follow from those of Legends. A timeline published in the magazine Dengeki Nintendo 64 places the two games in the same chronology and explictly mentions Trevor is Sonia's son.  However, it should be noted that this timeline is not official and that an article from Konami Magazine states that Legends doesn't follow Igarashi's chronology.  This is further supported by the fact that certain story details from Dracula's Curse don't match up with Legends, such as the fact that both games claim to depict the first confrontation between Dracula and the Belmonts.
This theory is based on the fact that there was no consistent chronology before Igarashi created his timeline. Before the release of Legends, both Castlevania: The Adventure and Dracula's Curse had attempted to establish an origin story for the series. Both games acted as direct prequels to the original Castlevania (video game) starring Simon Belmont. However, both games were developed without any coordination between the two teams, resulting in two different accounts of what occured before the era of Simon. Likewise, the background details in Legends don't match up with either The Adventure or Dracula's Curse. This leaves open the possibility that Legends was developed as a direct prequel to the original Castlevania without any regard for the above two games. There are some hints that this is indeed the case. Sonia is mentioned to be Simon's mother in several preview articles for Castlevania: Resurrection in Electronic Gaming Monthly. However, it's not clear if this is official information or a mistake made by the author. Furthermore, after Medusa is defeated in Legends, she is shown to have survived as her head is shown flying away. In the original Castlevania, only Medusa's head is fought, which implies she was decapitated at some point in the past. Thus, this little detail from Legends might allude to Simon's future confrontation with Medusa.