In ancient times, Brigid created a magical diary as a gift to humans.
Disturbance in Tokyo Bay
The diary was displaced in 1951 Tokyo, Japan, where it found its way to a film director, Ishirō Honda, who created the fictional creature Tagumo in his imagination as a representation of the destruction caused by the bombs dropped on his home, Hiroshima, during World War II. Once Ishirō took possession of the book, the diary itself drew Tagumo's storyline from Ishirō's imagination and then brought it to life. In the diary, Tagumo's story ends with the octopus destroying Tokyo.
After Ishirō was confronted by Zari and Mick on the recent unusual marine activity that Tagumo caused, he tried to throw the diary in Tokyo Bay in an attempt to erase Tagumo from existence, only for the book to find its way back to Ishirō. Charlie then explained that the book bonds itself to artists with creative minds, and that Tagumo wouldn't cease to exist until its story was played out in full.
Ishirō attempted to create a new ending to Tagumo's story, only for his pen's ink to be erased as soon as he drew on the book. Charlie also explained that the book was created to help artists express themselves, and that he can't create a happy ending as he didn't have a happy life.
Mick then bonded with the diary and wrote to bring Garima, a character in his original story, to life to battle Tagumo. The story ended with Garima vanquishing Tagumo and making passionate love with Buck (pertaining to Mick), which played out in real life. The Legends returned to the Waverider, where John Constantine kept the book in the chest of his possessions.
At some point, Mick stole the diary from John's trunk and continued to write Garima, using its power to turn her into flesh and blood once more.
Sara and Ava, to her surprise, saw Garima on the Waverider, to which Ava argued that letting Mick use the diary is dangerous. This led Ava to confiscate the diary as a safety precaution. As Mick confronted John for the loss and the two began arguing, she told them what she'd done and declared that the book was now officially Time Bureau property.
Stolen back by Mick
At some point after Ava confiscated the diary, locking it up in an "impenetrable" vault, Mick managed to steal the book back, secretly continuing to use it. When the Legends needed to finish Heyworld (a theme park featuring magical creatures) extremely quickly, Mick revealed to Ava and his teammates that he'd secretly taken the magical diary from the Bureau. He then lent the journal to Nate so he could bring Heyworld to life immediately.
- Magical bond: The diary is attracted to a person with a creative mind (e.g. artist, writer, etc.) and bonds with them, physically and empathically.
- Fiction manifestation: When the diary is bonded to an artist, it brings any story from their imagination into existence until the story ends. Other artists can interfere with the story by also bonding with the book.
- Enhanced creativity: The diary boosts the creativity of whoever it is bonded to.
- Limited Inspiration: Brigid's Diary can only help pull out ideas buried in the imagination. It cannot create ideas for the writer, only nudge them into letting out their creativity. Both Mick and Mona agreed that Mick had finally run out of ideas for his Rebecca Silver romance novels; Carjacked, his recent work, was notably showing signs of losing the "magic" his previous books did.