In theory, a truenamer's focus is pretty cool: more or less, when the universe was created, it involved the use of a hidden language, supposedly only known to the gods. This language, now called "truespeak," included the "true" names of everything that ever would and will exist. Only snippets of it still survive, and these snippets are what truenamers use. They identify the true name of a target and then modify it using the words of the cosmos. (Mechanically, this is shown by the existence of a Truespeak skill, which the player rolls against a DC to determine whether it works or not.) It has the same origins as spells like the Power Word spells, Holy Word and its variants, and Command, but truenamers focus on it full-time.
Utterances, their equivalents to spells, are divided into three Lexicons: the Evolving Mind, which make up the majority of options, affects creatures, the Crafted Tool, which comes somewhat later, affects items, and the Perfected Map, which kicks in at high levels, affects the environment. Additionally, Evolving Mind utterances can be said in reverse, meaning they create the opposite effect: an utterance that normally heals can be reversed into one that deals damage, for instance. This slots them into the role of a kind of pseudo-caster, focused mostly on buffs and debuffs and occasional utility, with enough tricks to give them a lot of things to work with. All in all, barring the unintentionally hilarious quotes of what truespeak sounds like ("shake the earth" apparently translates to "thremcheumalach-tura’abachnir"), a solid foundation, especially if you're a fan of The Earthsea Cycle. Unfortunately, once you get into the mechanics, things start going downhill fast.
The most infamous weakness to truenamers is the mechanics of the Truespeak skill, particularly for the "bread and butter" Evolving Mind utterances. These increase based on the opponent's CR, but said increase happens at a faster rate than natural skill investment (+2 per level versus +1), which means that unless the truenamer finds lots of outside methods to boost Truespeak, they're going to find that the longer they play, the less their abilities are going to work. For instance, the DC to affect a CR 1 opponent is 17: a truenamer with Skill Focus, max ranks, and 16 Intelligence can hit that about 70% of the time. The DC to affect a CR 15 opponent is 45: that same truenamer at level 15 with natural Intelligence boosts and no items is only going to hit that DC on a natural 20. This is worsened by the Law of Resistance, which means that every successful use of an utterance sticks an additional +2 on the DC for that utterance (which is also going to add bookkeeping to the mix on top of everything else). It's made especially problematic by the fact a truenamer still has to roll to affect their allies, which means that even your buff spells have a good chance of being resisted. Altogether, it ensures that a lot of truenamer turns are going to involve you screaming gibberish while nothing happens.
Because of this, it's borderline mandatory that the truenamer have access to specific magic items: a Amulet of the Silver Tongue provides a +5 (lesser version) or +10 (greater version) to the skill, which puts you somewhat back on the curve but still requires further investment to make your rolls reliable. Once you've managed to wrangle together enough items and esoteric bonuses (i.e. membership in the Paragnostic Assembly, the illumian's racial abilities, an item familiar, and/or the aforementioned Amulet), you should be able to succeed your Truespeak checks reliably, or even 100% of the time, bypassing the class's wonky mechanics entirely and effectively letting you use them all day long a la the warlock. Unfortunately, things don't really improve from there, due to flaws with the utterances themselves.
While most of your utterances are multipurpose to some degree, you can't take very many utterances, and there isn't an easy way to swap out older or less relevant ones. For how unreliable your utterances are, you'd expect them to be incredibly powerful, but they broadly aren't. The majority of utterances past 3rd level are severely undertuned for when you get them, stacking up poorly next to equivalent spells available far earlier (for instance, compare Reversed Essence of Lifespark, available at 14th level and giving the target one negative level, with Enervation, which is available at 7th level for wizards and gives 1d4 negative levels). The utterances often include spelling errors and other omissions that make them completely different from what they were intended to do: most notably, in the first print run of the book, the DCs for how Perfected Map utterances worked were not there, meaning they had to be added in errata, and the DCs for Crafted Tool utterances suggest that it's harder to affect a mundane sword than a +1 sword. The existence of the Law of Sequence, which prohibits you from having two of the same utterance active at once, just makes things even worse. Wanted to use Knight's Puissance to buff the fighter, and then reverse it to debuff the guy he's fighting? You're outta luck. And for the cherry on top, several utterances require saving throws, which are based on the truenamer's Charisma, of all things, so that's even more potential for a Truespeak attempt to do nothing.
In short, their editing and playtesting were non-existent and it shows. Notably, the first incarnation of the game's tier system refused to rate truenamers entirely, due to the class's mechanics being so broken that it's difficult to grade how powerful an average truenamer is. A well-optimized truenamer with the aforementioned Truespeak-maxing tricks is generally considered a low Tier 4 class, while a relatively unoptimized one drops all the way to Tier 6, hanging out with the aristocrat and the CW samurai.
Truenamer's mechanics aren't entirely wholecloth. It has its roots in, of all things, Star Wars D20's force system where force powers were all separate skills and needed random dice rolls to work. While this didn't have the constantly increasing skill checks, it did eat your hit points. Unlike Truenamer, that was worth dealing with because, despite how much of a clusterfuck it was, it was far more powerful than what non-force classes got. Ditching this sytem was one of the many changes Saga Edition made, replacing it with a much easier system of per-encounter uses with power determined by roll of a single Use the Force skill.
It's also believed that the aforementioned issue of DCs increasing faster than the player's natural bonuses was a testbed for how these things functioned in 4e, where things like the AC and attack rolls of monsters increase at a faster rate than the player's natural attack bonuses and AC, necessitating the use of feats, magic items, and special abilities to remain on par with them. The difference being, of course, that 4e was designed with this mindset from the ground up, rather than bolting it onto a system with a very different ethos.
Ways to make yourself feel better about playing this class
So you decided to play a Truenamer because surely it can't be THAT bad, right? You were wrong. Luckily, the utterances are just as poorly written as the class itself, so there are a few things you can do to make those 20 levels of suck almost worth it. Almost.
- The Incarnation of Angels gives a target the Fiendish/Celestial template, which makes them extraplanar, and therefore subject to Banishing! Where do they go? Who knows? But they're sure as hell not your problem anymore.
- Word of Nurturing, as written, allows you to affect "your target" and then lets you give "a creature" fast healing. Buy a rabbit, target it with the utterance for a DC 16ish truespeak check, give whatever you want fast healing. This works for a surprisingly high number of utterances because whoever edited Tome of Magic did not speak English.
- If you can somehow get your Truespeak check really high, Quicken Utterance allows you to add +20 to the DC to turn an utterance into a swift action. The odds aren't in your favor of succeeding, but even if that utterance fails, it doesn't eat your turn, so trying to use it every turn gives you a fairly decent chance to effectively double your actions.
- The reversed version of Greater Seek the Sky removes the target's ability to fly. Unlike the lesser version, it has an instantaneous duration, so that flight is gone forever if it succeeds. Ether Reforged (turns a creature ethereal) appears to do the same thing, which lets you trap a creature on the ethereal plane forever (or give everyone in the party a permanent Ghost Touch effect).
- Despite coming at 10th level, Rebuild Item is actually pretty good, letting you repair pretty much anything as long as it was broken in the last round. This gets fun when you realize how many single-use magic items out there require you to break them to activate them. There are even variant rules for "potion tiles", meaning this utterance lets you recycle potions.
- You have one utterance that gives a +5 bonus to skill checks, another that gives a +10 bonus to your next Knowledge check and the ability to count as trained in it, a free +3 to Knowledge checks, an Intelligence focus, all Knowledge skills as class skills and not much else to spend them on, and you're probably part of an organization that gives a +5 or +10 to Intelligence-based checks. By 3rd level, you should be able to narrate the GM's own campaign to them... you know, if you want to be the exposition dump.
- Truenamer can eventually mimic Gate, one of the most broken spells in the game, while ignoring one of its few restrictions in the form of XP cost. This only comes online at level 20, though, making it sharply limited in any kind of practical campaign.
Most interesting thing about Truenamers
So overall, truenamers are shit as far as classes go, but truenamers have one very interesting feature to them. It's mentioned in a side box that some people's true name gets harder and harder to say the older and more experienced they get, noting that many people who show this 'phenomenon' become leaders, heroes or villains. The truenamers think that this is because the universe takes an interest in people as they achieve more, become more powerful and more important to the universe, their true name becomes harder to say. But the Tome of Magic says it's because the person is gaining levels and hit dice, thus their DC to speak their true name goes up as well. To the NPCs to which the world is real, with hit dice as an abstraction, they only have a, quote, "vague sense of what's going on."
Stew on that for a second: the NPCs of a game, are able to detect to some degree the influence of the 'player' and are able to sense when that 'player' gets enough 'XP' by shivving enough crabs to make a person in the world stronger by go up a 'level'. We are not sure if that's fail or awesome, you decide. This isn't the first time D&D acknowledged levels were an in-universe concept, though, as Trap the Soul in core mentions it is possible to "research" how many hit die a character has.
Onomancer, or Utter Hubris: the 5e Truenamer
Wizards has had a lengthy love-affair with trying to make a "metamagic wizard" in 5th Edition, despite the fact that metamagic is one of the sorcerer class's few remaining good things in an edition that stripped almost all of them away, plus the wizard is already being a spoilt brat in terms of class options, spell list, class features, and almost everything else in comparison.
So, some probably-industrial-grade-drug-addled designer thought, why not mix that hubris with the hubris of trying to design a 5e version of one of the worst classes in the game's history?
In the October 2nd, 2019 Unearthed Arcana, a UA otherwise full of solid (if somewhat unremarkable) player options, Wizards featured the Onomancer. We'd say this is because Wizards of the Coast think you are stupid and don't know Latin or how to Google, but it all-but calls them "truenamers" in the opening text and sidebar.
The Onomancer is a wizard whose entire schtick revolves around magically securing the "true names" of various creatures, via a 2nd level ability that lets them force a creature to make a save, and upon then applying bonus effects to every spell they cast on a creature whose true name they know.
Unfortunately, as with some other updated classes that were infamously terrible, Wizards decided they needed to keep that janky power-swing that characterizes the previous class for a semi-authentic truenamer experience.
First, you can only use your Extract Name class feature a number of times per long rest equal to your Intelligence modifier (so, a maximum of five, ever), and if they make the save, you can never use that ability on the target again. Ever. Yep, a feature so limited you are heavily incentivized to save it for high-powered boss monsters, but which, when used on high powered boss monsters, has a chance to fail permanently. And that's not even bringing up that many high powered boss monsters have either high Wisdom and proficiency in Wisdom saves, Magic Resistance for advantage on saves vs. all magical effects, and for some, the ability to choose to succeed on a number of saves every day. Some have more than one of these at once.
This means many players and parties won't be able to reliably use the built-in class features... and that the GM will have to adjudicate according to the intentionally-vague "true name" sidebar that opens the class if the party tries to extract that name via other means, like interrogating underlings. It also means that the onomancer's whole schtick doesn't work on anything without a language or higher intelligence, and therefore without a true name. Have fun battling some of the high-level elementals, monstrosities, anything with an unaligned alignment!
And they'll have to, because, aside from a few extra proficiencies and the ability to cast bless on allied teammates a couple of times for free, every other ability in the class relies on adding bonus effects to spells once you know something's true name, so anyone playing an onomancer will want as many of them as they can get.
Worse, those effects are often stronger versions of stuff sorcerers can do with metamagic, including over-the-top powers like dispelling a magical effect on the target of the spell using a similar system to dispel magic, firing spells that don't require line of sight, or just offering buttloads of temporary hitpoints or extra damage (irresistible force damage, of course). And unlike sorcerers, this system doesn't cost extra for some effects vs. others, at the "cost" of not being able to spend spell slots to recharge them.
Also, at 14th level, they can make any spell that deals damage to a creature whose True Name they know either psychic or force for free, because letting the wizard turn all damage into irresistable damage worked so well for the Lore wizard, even with some built-in limitations.
Even if it's not as obviously broken as the truenamer, the onomancer is still janky, based around a poorly-designed mechanic that offers incredible power or absolutely fuck-all in a feast-or-famine system, and one that actively devalues a class that's already suffering from power discrepancy issues this edition. Avoid. Here's hoping they don't ignore all feedback and spite-print it in the next player options book, like certain other mistakes from Xanathar's Guide to Everything.
Update: The Onomancer Wizard has been confirmed to be dropped from further development in the Subclasses Revisited UA from May 2020 due to lack of appeal. Consider this bullet to be dodged.