The first book that isn't a journal. Historical information, right here.
Author information: Tilenu Uvelar is among Tamriel's authorities on assassins. She personally interviewed most of the world's famous assassins, including Bathes-in-Blood.
Tale of Scales, by Tilenu Uvelar
Chapter 1: Steps-in-Shadows
Any Argonian hatched under the sign of the Shadow is deemed a Shadowscale. However, for the last several decades, the Shadow sign has marked no births. That is until Fredas, 23 Frostfall 4E 214. This is the date of birth for the latest, most famous, and possibly most talented Shadowscale in recent centuries.
Born Steps-in-Shadows, she was brought to the King of Black Marsh as soon as she was able to walk. This is where her training began. She was trained in the arts of assassination. That is, archery and the art of knife-throwing. She quickly revealed herself to be a prodigy at these. She excelled above even her teachers in the ways of murder, outshining all other assassins brought to her sessions. By the age of nine, she was well capable of killing anybody she wanted.
The King only had one more thing left to test, her loyalty. Steps-in-Shadows had little relations with anybody else. The closest bond she had made was probably with her instructor, whom she had respected deeply. The King brought them both in his chambers and ordered her to kill her instructor. She was skeptical and unsure for the briefest of moments, before nodding her head. She grabbed her favorite knife, one she always kept on the inside of her boot. The blade shook in her hand for just a second, then she readied for a still shot.
If we were examining Steps-in-Shadows' psychological state, this would probably be the defining moment where she completely lost what she had left of her morality. She flung the knife with such aim, such speed, that it pierced her instructor's eye and gouged into his brain, an instant death. She had done the deed. By the personal account of the King, there was a certain aspect in her eyes that went wild right then.
As the blood began to pour from her instructor's wound, she crouched down near him, embracing his lifeless form. She rubbed her hands together in the blood of her old teacher, not willing to let go of his essence. She lay there, rubbing his blood on her clothes and face for a good five minutes or so. She was pulled away by the King, and this earned her her new name, Bathes-in-Blood.
Chapter 2: Blood of Dragons
After that day, she was even more effective than before. When she was offered to the Dark Brotherhood, her first assignment had her go to Skyrim to kill the Dragonborn. Her plan was to simply corner her in her house in Markarth. She spent most of her time in there, tending to what was the largest library in all of Skyrim at the time.
According to Bathes-in-Blood herself, she sneaked in relatively easily. She was bewildered at the amount of books in the house, mostly because none of them were the same. She made her way into the master bedroom to find the Dragonborn, Libario VI, asleep in her bed. She took a moment to gather her surroundings, but then heard a voice, "So are you here for a book or are you here to put that dagger in my neck?"
In my exclusive interview with Bathes-in-Blood, she reveals the entire battle, from Librario's menagerie of powers to her eventual death. The first thing that Librario did was unleashed an Ice Form shout, freezing Bathes-in-Blood in her tracks. She doesn't recall being frozen, but when she recovered, she found that her opponent had transformed into a vampire lord. She remembered dashing out of the room, only to be followed by the vampire. She stood in front of a book case as to prevent any terribly strong spells. Surely someone as devoted as Librario VI wouldn't ruin some dozens of books. The assassin's instinct was right, and Librario didn't attempt to cast any spells. Instead, she forfeited her vampire form and donned her armor, a massive set of daedric armor glowing with the power of several heavy enchantments.
Librario stormed upon her assailant, not losing a moment. However, Bathes-in-Blood was fast and able to dodge the blow. She threw a knife, but it only glanced off of her target's armor. Librario looked at Bathes-in-Blood for a few seconds before asking, "You're with the Dark Brotherhood, aren't you?" Librario held up her sword, but her attacker was silent. Bathes-in-Blood recalls the next speech as the sole reason she regrets killing the Dragonborn. Librario stood strong and said "I've fought through swarms of Daedra. I killed Alduin the World-Eater so that Tamriel can continue to thrive. I can command the will of the dragons that Skyrim is so afraid of. I have visited Apocrypha to slay the first Dragonborn and I've come out unscathed. And you believe you have the reason and ability to slay me?" She then proceeded to pull a book out of a drawer nearby. Bathes-in-Blood didn't know it at the time, but now identifies it as a Black Book, a portal to Hermaeus Mora's realm of Apocrypha. Librario opened the book, casting it's gaze onto Bathes-in-Blood.
She was unable to describe the experience of being pulled into Oblivion, but she described the place as a mass of books and dark structures. When she had come to her senses, she found that her opponent was there with her, commanding some hulking monstrosity to attack her. According to Bathes-in-Blood, the beast was surprisingly easy to take down using her throwing daggers.
The moment of death came to the Dragonborn only a second later. Bathes-in-Blood drew a single dagger and threw it as quickly as she could. The Dragonborn let an Ice Form shout out, freezing the dagger just before it hit her straight through her throat. Bathes-in-Blood was able to dodge the shout and escape from Apocrypha using the Black Book. When she returned to the library, she scoured through the house to find five of the Black Books, and stole away with them. The guards didn't suspect anything. The Dragonborn's house was a library with complete public access. Visitors were common.
Within the day, Vlindrel Hall was closed down for investigation.