The Labyrinth Worldbook by Kobold Press is a source book for Tales of the Valiant or any DnD 5e variant. This is my review of chapter 4: Heroes of the Labyrinth. It's the player options chapter.
As I write this, I'm just starting up a Labyrinth Adventures campaign with one of my gaming groups. We spent a session building characters, and I have to confess that not one of them went for any of the options in chapter 4 of the Labyrinth Worldbook. I don't think that's a reflection of the options, because the same thing used to happen when I handed out the Sword Coast book. It seems to me that players either don't know they can use anything but what's listed in the core rulebook, or else they know they can use ANYTHING. The players in the first camp restrict themselves to the rulebook, and the players in the second forsake all rulebooks and find exactly what they want on the Internet. It's a thankless job, writing player options in subsequent source books.
That said, there are some good player options in this chapter, and if I were playing in your Labyrinth campaign I'd choose from this chapter.
Obligatory subclasses, and not all of them are tightly bound to the Labyrinth.
You hear the whispers of those unjustly or violently slain before their time.
The Forsaken barbarian is aided by the spirits of the fallen, and those spirits grant you bonuses. At level 3, the spirits add either radiant or necrotic damage to your attacks while raging. At level 7, they consume a condition that is effecting you (you can just end a condition as a bonus action!)
Deception and subterfuge.
This makes a rogue-ish bard build easy, with no need for multiclassing (well, unless you want the actual mechanical benefits of a rogue.) At level 3, you get the Slippery Strike feature that allows you to make an attack and then get away without an opportunity attack. At level 11, you can magically teleport 10 feet times your proficiency bonus.
In short, it evokes the sense of movement and uncanny dodges that a rogue has access to, without actually replicating them. Combine this subclass with careful selection of spells, and I think it make for a really interesting thief.
Creativity and invention.
Craft an animated item, gain spells like heat metal and shatter and column of cogs (a new spell from the Labyrinth Worldbook.) It's clockwork steampunk, but powered by divine magic.
Seek out the unknown, educate the unaware, and preserve all forms of knowledge and enlightenment.
Grants spells like augury and clairvoyance to suggest that the character has preternatural awareness of what's going on and, more importantly, what's going to happen. This can get tricky as a game master, but I'm pretty generous to clerics and religious characters. I often just assume that, given a sufficiently devout character, augury is always on, dropping little hints from their god here and there as appropriate. I'm not sure I could make this cleric feel very different from any cleric with augury, but as long as there's only one cleric in the party it arguably doesn't really matter. Still, I consider this subclass objectively one of the weaker designs of the player options (but subjectively, I like it OK.)
Beguile, manipulate, and distract.
A true illusionist at last! I've been trying to build a pure illusionist for years. I've technically achieved it, but the result has been pretty useless mechanically because most illusions don't deal damage. Using a cleric as the foundation of an illusionist, however, is pretty brilliant. You may not deal a lot of damage through your illusions, but you at least have all the healing and buffing abilities inherent to the cleric class. Thanks to some clever design, the Trickery Domain cleric gets some useful abilities, like invisibility that doesn't end when you attack or cast a spell (Greater Ruse at level 11.)
This is definitely my next character build.
Tap into the collective unconsciousness and perceive secret psychic and spiritual connections.
An obvious choice for any member of the Dreamers faction, this grants a bunch of new dream spells from the Labyrinth Worldbook. You can also use Wild Shape to create an Energy Link between creatures within 10 feet of you, which basically just grants you and your allies a 1d4 bonus to attacks and saves. At level 7, you can get bonuses or impose penalties based on the number of enemies on the battlefield.
Survive and thrive in any environment.
Speed and mobility increases, damage bonuses, and the ability to treat any level of Exhaustion as 1 less than it actually is.
Clockwork companion.
At 4 pages, this subclass basically introduces a minigame about building and leveling up a clockwork companion. The companion is undeniably cool. By level 11, you can add a flamethrower to it, or load it with a healing serum so it can give you an HP boost when necessary.
Kindle and amplify light wherever there is the threat of darkness.
Throw punches of radiant light, scorch your foes with sacred fire, nuke an army with a searing corona.
Freedom, nature, light, and justice.
An obvious choice for a member of the Elders. This grants you spells like fog cloud, spike growth, wall of stone, fly, and freedom of movement, and resistance to various elemental damage types. At level 15, you gain a sort of Wild Shape ability that allows you to transform into a primordial elemental that's pretty powerful.
A ranger of the spaces between.
One of the most Labyrinthine subclasses of them all, this is ideal for the player who wants to play the character who's more at home in the spaces between worlds than within any one world. Up to level 15, this means concentrating on using Survival to track planar entities (Celestials, Elementals, Fiends, Fey, Outsiders) and gaining spells that deal with dimensions (such as blink and dimension door and the new pocket portal.) At level 15, you can create a portal to anywhere you know in the multiverse.
Imposter syndrome.
A variation on the standard rogue, with some benefits to defense, free use of message, and a slight bonus to attacks at level 15. Not my favourite subclass in this book.
Embrace what lies beyond comprehension.
Control and manipulate darkness. You can use darkness as a gateway for teleportation, you can cause it to attack or confuse your enemies. This seems like it'll open up a lot of creativity for a player, and I think it'd be fun to play in a game with an Umbral sorcerer.
A patron generous with both their gifts and their expectations.
It feels like a combination so unlikely that it's maybe even dangerous, but this subclass provides a warlock with a greater Celestial patron. I really like the flavour here, but admittedly it does play fast and loose with the typical warlock tropes. I don't think a player looking to build a warlock is likely to go for this one, but a player looking for something curious is gonna love it.
Those who walk in the light have no hope against the things that lurk in the dark.
As much as the Celestial subclass challenges the flexibility of the Warlock class, the Void Saint perfectly reinforces it. Your patron is a Void Saint, which grants you some boosts to magic (in terms of range and recovery). You gain some domination with damage spells (command, confusion, ray of enfeeblement, and the new void strike.)
If you're looking for a classically dark warlock, this is a good choice.
Unlock the dark mysteries of the Void.
Wizards are cool until they start dabbling in the dark arts, or at least that used to be the mood of high fantasy. The real world has moved on from classic interpretations of "black magic," and it largely seems that any magic is good magic. Players hardly notice when a heroic wizard goes down the path of necromancy. So Void magic is the new necromancy. It's the magic that uses the force of annihilation, of literal non-existence.
The thing that's missing, for me, from Void magic is the roll a d20, and on a 1 you cease to exist rule. If the Void is meant to be big and bad and scary and threatening to both heroes and villains, I want to feel that in my campaign. I'm absolutely making that a rule in my campaign, anyway.
Bored with playing humans and dwarves and elves and kobolds? Well, there are new player lineages!
Additionally, there are some variants of classic lineages:
Heritage defines your cultural inheritance, your family's origin, and possibly your default assumptions about how the world(s) works. Mechanically, they grant languages and proficiencies and maybe a quirky ability here and there. For role play, they help inform how your character acts and reacts to certain situations.
A background hints at what you did with your life before becoming an adventurer. I love backgrounds in 5e, and I'm a firm believer that there just can't be enough of them to choose from.
There are also backgrounds specific to each heroic faction. These assume you've grown up in the Labyrinth, and have integrated into one of the societies that exists between the realms.
There are a few new talents, which are essentially Feats. There are some for magic users and for martial fighters, there are some technical talents, and also a vehicle-based talent.
There's over 40 pages of player options, which makes this book verifiably as useful for players as for game masters. Everybody needs lore, and there's been a bunch of that in the book (and there's more in chapter 6.) Players need options, and chapter 4 provides plenty of that. The next chapter adds new spells and magic items and, maybe most interestingly of all, vehicles. When they labeled this a worldbook, they were being modest!