I think this is much improved over your earlier versions. I only have a few minor edits, which I've noted in blue below. Otherwise, I think this is all tightened up and ready to go. Nice work!
Dammit I hate this stupid iPad. Anyway...
SYNOPSIS: THE PERSONAL ACCOUNT OF DARIUS CARLYLE
[The following has been extracted and novelized from DARIUS CARLYLE's personal journal, dated 1875-1889]
When Darius returns to Wolfstone Manor following his grandfather's death, he finds himself inheriting the house, estate, and—most importantly—the library, and its hoard of esoteric lore. After a century and a half steeped in slaughter, Darius is grateful for a home of his own at last. But he continues covertly hunting werewolves, lest complacency get the better of him. I think you should combine this paragraph and the following one, as well as cut the sentence about Darius continuing to hunt werewolves. I would set it up basically like this: Darius is grateful for a home, he meets Kate, they get married, get pregnant, Darius is ready to settle down." Then jump immediately into Darius being pulled into the Jack the Ripper case. This construction sets up a good emotional conflict, in my opinion, for Darius. Family and a normal life vs. his calling to hunt werewolves. Plus I think it would just flow a bit better if this were one paragraph instead of two. It's a bit choppy as is.
Then Darius happens upon the eucatastrophic Is eucatastrophic a word or a typo? Legitimately asking, as I've never heard it before and couldn't find anything in the dictionary. KATHERINE HARROW.
By the autumn of 1888 I think you can remove the date here and below, when Tobias is born. You have the date listed at the beginning, and including more dates just makes it seem like the reader needs to keep track of them for some purpose, but they don't really need to in the synopsis., Darius and Kate are married and expecting their first child. Meanwhile, Jack the Ripper has London choking on its own terror. Then the inspector in charge of the case writes Darius. He isn't even close to finding his man (if man he be at all), and is in need of Darius's particular expertise. Though at first unwilling, Kate encourages Darius to take the case, and even follows him to Whitechapel in London. Though dismayed, Darius cannot help but be in awe of Kate's innate talent for subversion This sentence feels superfluous. When Darius uncovers that the 'Ripper' is a conspiracy manipulated by CAIN–a so-called 'True Ancient' werewolf with an incomprehensibly personal vendetta against Darius's family–the unthinkable happens. Cain kidnaps Kate. Though the Ripper is slain and Darius manages to save her, Darius is shocked to discover that the werewolf has cursed his dearest Kate with an arcane form of lycanthropy.
With each full moon Kate's condition grows progressively worse. Curiously, however, her pregnancy is not affected in the slightest, even when she undergoes the Change. On 20 April 1889 Kate literally rips their firstborn son from her womb, torn between the pangs of labor and grotesque convulsions of metamorphosis. After countless failed attempts to cure Kate–including a Catholic exorcism–Darius finds an ancient Wolven ritual that may save her. But it requires the blood of the werewolf that turned Kate.
Darius manages to snare Cain, but the ritual fails, and Kate helps Cain escape through the library. Unhinged, Darius tortures any werewolf he can find for information–while torturing himself for his failures. Eventually he locates Cain, and his dearest Kate. The only other possible cure he has found requires Darius to kill Cain. Certain he is going to his own death, Darius sorrowfully leaves his son in the care of his last trusted friend in this world.
Then Darius meets Cain at Stonehenge.
The final entry of the journal is a letter Darius writes to his dearest Kate, in which he grievously recounts his final stand against Cain, and his beloved. Cain goads Darius's bloodlust as the Wolven cuts through the pack of werewolves, each kill swifter than the last, until it is intimately instinctive. But Darius only just restrains himself when his dearest Kate is the last left. She attacks Darius at Cain's command. Torn between love and bloodlust, Darius pleads with Kate to hold on to her indomitable humanity. Darius, using mystical tactics, manages to subdue Kate, and turn his rage towards Cain. But at the sound of a sudden snarl behind him Darius, on instinct, turns and strikes–driving his blade through Kate's heart. As silvery fire consumes her before Darius's eyes he is completely, utterly broken. And as a final taunt, Cain reveals to Darius that he stole a book from Darius's library when he was captured for the ritual, which Darius had never known of: a book of profane werewolf magick, that contains the actual cure for lycanthropy. Darius lashes out at Cain but the werewolf stays his hand, telling the Wolven to savor his vengeance a while longer. Cain then vanishes, leaving Darius alone amid the ashes of his dearest Kate. Wracked with grief and blame, Darius ends this final letter to the love that he killed, writing, "I'm sorry that I loved you," and vowing to right his wrongs. The journal ends here, Darius's fate after writing this letter unknown.
Some time later the journal finds its way into the hands of Darius's son. On the inside cover an inscription reads:
If you ever read this, I hope you can understand; even if you can't forgive me.