The Erudite (knowing a lot) is a psionic class in 3.5e Dungeons & Dragons. It is also one of six most powerful classes in all of 3.5e, but not as it appears in the main book. In practice it is the Wizard to the Psion's Sorcerer, with a bit of Archivist thrown in. The Erudite is something of an unknown class, being muffled away all the way in the back of Complete Psionics, a book Psionics fans hate, alongside the Epic rules for psionic characters. Luckily you can find it for free in the book's preview excerpts and spare yourself the hassle of actually remembering Complete Psionics exists.
How to use it
The Erudite works very much like a regular Psion: it has power points, uses them to cast its psionic powers, can interact with psicrystals and the fun stuff that entails. Its BAB is only so-so (10/5), has 6/6/12 at saves at level 20 and only d4 hit die. It has only limited skills and gains bonus psionic feats at levels 1, 5, 10, 15 and 20. The Erudite can also use only a limited amount of unique powers per day. This means that while at first it can only cast a single level 1 power twice a day, later it can use up to 11 different powers per day. The main difference between it and the Psion is that the Psion learns powers on its own accord, while the Erudite has to learn them from somewhere: either from an item containing this power or from someone who knows such a power. The target has to be willing to do so: meaning that if you want to learn from someone you must either talk them into it, use some kind of spell to make them do it or knock them out (because unconscious psions can't say no).
When the Erudite pulls such a power from a source (which requires two Psicraft checks on DC 15 + power level if the source of the powers fails a Will save DC 13 + Erudite Int bonus) he can either use it immediately but only once, OR he can meditate on the power for 8 hours and add it permanently to its repertoire. Such power comes at a price however... at 20xp per power learned per Erudite level. While this is something of a notable commitment for a lower level character, at higher levels this becomes almost free. This means that learning single powers will work, but you can't pilfer an enemy's mind completely empty if you don't stick around their unconscious body for a few days. And while this does not damage the minds of anyone you take the power from, items containing the spell will lose it.
How to break it
Now, while this would be pretty useful on its own, in 2007 a web expansion was released regarding the Erudite. This gave birth to the Spell to Power variant of the Erudite, which allows them to do the power learning thing with arcane spells. This means that an Erudite can not only learn psionic powers, but also ALL Wizard, Bard, Assassin and Wu jen spells and use them as power. Dragon #349's feat Chameleon Crafting also lets the Spell to Power Erudite get all divine spells too, like an archivist. The sheer diversity that comes with such a repertoire makes the Erudite a unique power on the battlefield and outside it.
Now, casting your spells is always done at the minimal level when dealing with "X damage per caster level", but for every point you spend this goes up by 1. This can be costly if you try to go for max damage spells, so keep this in mind. You can also choose to pay 2 points extra to pay for ANY material costs that come with the spell, so if you want to have some fun with Fabricate, knock yourself out.
Some of you may be saying to yourself, "Holy swizzle-dick, Batman! What in the world would NOT be to love about this class, especially with spell-to-power meaning I can learn literally every power and arcane spell in the game?!"
Well... hold up a second there, Bucky, it's not all cupcakes and rainbows out of a unicorn's fart-box here. There's some things to discuss first.
First up is that the RAW version of erudite, as of the end of 3.5, only allows you to manifest a very, very, very few powers each day. Take a look in Complete Psionic on table 6-6 which lists the erudite class stuff. See that column that says "Unique Powers/Day"? That's right: you can only choose that many powers in your repertoire to manifest each day.
So, for example, if you're a 5th-level erudite, and you have 12 1st-level powers, 7 2nd-level powers, and 4 3rd-level powers, you get to pick just 3 of any of those powers to manifest all day long. That's 3 out of a possible 23 powers. Look at that 20th-level row for erudite. You can only ever manifest a total of 11 powers, and even with funky epic progression, you'll always be boxed into a relatively smaller number of powers than your repertoire would indicate you can use.
Even after all these years, a lot of people on various forums debate this issue. Some think this is a perfectly acceptable price to pay for phenomenal cosmic power: you can only ever tap into some of that power at a time. Others fall back on the original erudite rules in Dragon #319 (pg. 46), which was written to say you can manifest unique powers of each power level, on par with a wizard (for example, in the magazine version, a 5th-level erudite can manifest a single 3rd-level power, two different 2nd-level powers, and three different 1st-level powers). There are others who say that both of these extremes are somewhat undesirable, so they houserule it somewhere in between (double the number of unique powers allowed by Complete Psionic, allowing a number of unique powers equal to an ardent's powers known earlier in that same book, or other crazy parity schemes).
But RAW? Erudites are fucking boned when it comes to trying to toss off a dozen powers. Sure, they can... but only a dozen of the SAME power. Note, however, they can still use psionic items with powers to make up the difference; because they can learn all the things, they can also activate all the things, which means you don't have to manifest it yourself if you got your local psionic artificer to make it for you.
Second disadvantage here is XP. Oh, I know some asshats will chant "XP is a river", like the properly brainwashed fuckwits they are. Yeah, tell you what, chief, you find a DM who allows you to totally break the fucking game by earning 1,000 XP more than the rest of the players do, and I'll one-up you with an artificer with that same amount of XP to build things (since only the really smart powergamers know that XP isn't the river, money is in an infinite multiverse with interplanar markets to tap into). Hell, forget that for a moment and realize that buying all the powers in the game still means you'll never get to the 9th-level badassery of stuff like gate (which gives you access to wish and miracle cheaper, usually also with something that can hurl so many SLAs and actual spellcasting that the DM is now having to fight back against his own Monster Manual), shapechange (all the brokenness of polymorph, plus all the special abilities that made not having them marginally less-broken for polymorph), or even wish (which is the least effective spell at that level for what it costs you, but hey it's still a possible IWIN button in a pinch, plus you need several of them to boost your inherent bonuses to +5, something a single efreeti can't really accomplish on their own).
The point here is that XP isn't a river, it's a fucking faucet and everyone gets to drink from it, not just your erudite. That does not mean you shouldn't learn more stuff; it means you actually need to think a moment before picking up some new power. Do you really need it? Can it do all the things? If not, what is so clutch about that power/spell?
Which brings us to the third disadvantage that nobody thinks is a real thing until the DM says so: you can't actually take every spell as a power. You learn spells as if they were powers not on the main psion/wilder list, meaning you can only ever learn something one level lower than your top power level available (thus, no 9th-level spells as powers without epic manifesting).
But more than that is the cooked-in limitation of not being able to learn any spell that allows you to recall/recast another spell. Due to technicalities, this takes away limited wish, energy transformation field, and anything funky that recalls spells (like Rary's mnemonic enhancer, for those of you trying to use powers to make your wizard side uber). Other spells suck because they count against unique powers per day AND require you to "cast" addition uniques to fire off, such as the arcane fusion and spell matrix spells.
However, not all is lost. The various shadow conjuration/evocation spells are just perfectly legit for you to take, because they emulate other spells; each casting of a shadow-based spell allows you to pick the emulated spell at the time of casting. Oh, and arcane spellsurge, which reduces casting times on future spells, that's kosher (because it only changes the effects on casting other spells, it doesn't actually recall/recast anything on its own).
Other stuff may be DM fiat. Transparency rules mean a mental pinnacle/dweomer of transference loop may not work as intended, but if your DM is cool with it, go for it. (Protip: if you're a DM, shit-can mental pinnacle and close the loop. If you allow this, you may as well allow efreeti wish-looping at around 12th level and admit you're going to have to restructure your game around the new "normal" of power.)
TL;DR Erudites are pretty fucking powerful, but they're still not as god-mode as clerics, druids, and/or wizards usually are. So pick good spells as powers. The polymorph, shadow, and other "multi-tool" spells are your friends.
Cerebremancer (aka How To Actually Break Your Game)
For the most part, any decent optimizer in the 3.5 community will tell you that theurgy (the merging of two spellcasting progressions) is usually shit. Like, a fresh pile of steaming feces. You end up sacrificing spellcasting progression on both sides for the dubious benefit of more spells, and then most people don't even really optimize themselves for that.
Which doesn't mean you can't win. Eldritch theurge (cleric/warlock) is well-known for breaking much of that idea apart, and ultimate magus has a whole Goddamn church of heresy built around the action economy it creates with a sorcerer/wizard combo (made worse with certain Dragon Magazine feats that double-dip CL increases).
Likewise, the cerebremancer using erudite as a chassis can potentially break action economy. See, with stuff like arcane spellsurge and celerity, as well as a few ACFs and feats cherry-picked here and there to help it along, you too can become the Spellcasting Speed Demon. Mix in some powers that can be used to stack on effects to the spells you used, and it gets pretty silly. Your DM may get dicey about you getting a quickened power and a quickened spell per turn, but even if you don't, you're still getting faster spells than normal spontaneous metamagic normally allows. One quick example would be to use quickened arcane spellsurge as a power, then you can fire off a summon monster and a standard-action buff spell (mage armor, bear's endurance, haste, etc.) as a swift action, plus you still have a move action you haven't taken yet.
Never mind the fact that you could be a lot of spellcasting classes that have SAD with erudite: focused specialist wizards, beguilers, and wu jen are all equally viable (but if you take warmage you will be judged a heretic). You can make a focused conjurer who sacks evocation, enchanting, and necromancy, and the erudite can fill in all those banned schools effectively (possibly without even learning the spells for evocation and/or enchanting, since psionics does better in those categories). You can make a transmuter who focuses on polymorphing, and even if you take a monstrous form that doesn't allow spellcasting, you still have a mind to use your psionics with.
If your DM is silly enough to allow you to play a psychic theurge from The Mind's Eye web enhancements (which are pretty officially legit, more so than most DMs consider Dragon Magazine material to be), you just won the game: play an archivist/erudite/PT, and you can learn 99% of the spells in the entire game made available to you. This isn't for "free", of course: you have to become a fucking miser with time, XP, money, and possibly other resources, to the point that the most munchkin gamers may start giving you the side-eye. But if you can claw your way past the deficit in casting/manifesting levels, you will more than make up for it with the ability to be 20 different things for the party, based on what they need and having 8 hours of rest to become that thing.
Really, though, if you're going this far, go ahead and grab Leadership and a psionic artificer cohort, and just become the new God-Emperor of D&D...
The third party Psionics port would get a bunch of unique additions through the followup Psionics Expanded, which introduced “Advanced Disciplines” for psions. Advanced Disciplines are variant disciplines with different powers but still user of the same discipline with all that entails that isn't explicitly swapped out, more or less like Focused Arcane Schools option Wizards got in Advanced Player’s Guide. The generalist discipline got an “Erudite” option that works just close enough to Complete Psionics Erudite to tell its inspiration. In exchange for two of their powers known and the discipline’s bonus feat at levels 2, 7, 12 and 17, they can prepare any spell from the general psion list they have previously used via a power stone (psionic scroll), and swap that prepared power to another such one each day. This is just sorta OK. While inability to change powers to meet the situation is one of a psion’s weaknesses, the low rate of getting floating powers, cost of powers known, and being restricted to non-discipline powers mean it’s just an OK option. How difficult it is to obtain a Power Stone is also a serious concern, as even GMs that run psioncs and like that will keep them less common than magic.