TL;DR: This is a mostly middling, occasionally decent, adventure novel with only superficial connections to the world of Dark Souls. If you want dark fantasy adventure there are much better options, and if you want immersion in the world of Dark Souls this probably won't scratch that itch.
The novel starts the same way all Souls games start: a corpse awakens somewhere after who knows how long with no memory of himself or the larger world, but he's compelled to adventure as if by some unknown force. Soon he's confronted with mindless undead out to kill him, and kill him they do. He reawakens and eventually bests these first foes only to quickly encounter bigger, stronger foes to kill and be killed by. This is the first chapter or two of Masque of Vindication. They are unfortunately probably the best part of the book as things go downhill from there.
The protagonist, who recalls his name is Ferranos, goes on a grand adventure with a company of other heroes in an effort to revive the old king and restore order to the land. The plot follows a very simple video game-like structure: the king's three legitimate children each hold a key to his tomb, so our heroes, including the king's bastard son Balarion, visit each of them in turn, completing some task on their behalf in order to obtain the keys. Over the course of the story characters drop in and out of the adventuring party as the plot takes us from region to region, skirmishes are fought and won (and sometimes lost), the adventurers grow stronger and more experienced, and the world is fleshed out (but not to a great degree).
At just under 300 pages, Masque of Vindication is a short dark fantasy adventure. There's magic, visceral descriptions of battles, and fantastical locales. However none of these things are specific to Dark Souls. The elements taken from the games are few and superficial: the company rests at bonfires where the undead among them can respawn if killed; several characters posses what is effectively an Estus flask, allowing them to heal near instantly; and killing their enemies awards the characters "experience," never referred to as souls, but allowing them insight into the enemy's life, thoughts, and the state of the world.
One of the biggest things that makes Souls-like games so captivating for me is the sense of isolation in a desolate and hostile world. You are a nameless, silent grunt alone in a world where humans are almost nowhere to be found, and a variety of strange creatures want to kill you. There are very few friendly faces to be found anywhere, and almost no places of refuge. In Masque, Ferranos is joined by four or five companions for almost his entire journey. With one exception his companions are living humans. They converse, sleep, and eat. Their travels take them to several large cities and villages, all teeming with life. Their battles, with little variety, are either against humans or generic "Hollows." It doesn't really feel like Dark Souls. It feels like a generic dark fantasy that borrowed a few surface-level elements from Dark Souls. Perhaps capturing the spirit and feel of Dark Souls in novel form isn't really possible.
But not capturing the feel of Dark Souls is not this novel's only problem. The dialogue is clunky, with characters repeating information they already know and that's already been established, antagonists stating their intentions and motivations in awkward exposition dumps, plot lines that go nowhere, and at least one not-so-subtle allusion to modern U.S. politics. This book also handles women characters quite poorly. Most women in the book are essentially sex objects, and vain about their appearance to a ridiculous degree. One even has a literal mental breakdown when she realizes she's not the most beautiful woman in the world. Finally, the ending is left open-ended allowing room for a sequel, but the book doesn't really earn it, so I'm mostly left feeling unsatisfied.
If you want dark fantasy adventure read The Black Company. If you want Dark Souls play Dark Souls. I'm not sure there's a good reason to read this.