With no Flaws, and moderate cheese
This version does not assume any variants from Unearthed Arcana are in play (though fractional saves get mentioned because this variant simply makes much more sense—it does not affect viability here). The cheese level is relatively low considering the goal, entering a prestige class “only” one level early. Though ur-priest is arguably cheesy no matter what you do with it...
Human Warlock 1, Precocious Apprentice and Invisible Spell
This gets you an arcane caster level, least invocations, a 2nd-level arcane spell, and a metamagic feat. It also gets you a Will save of +2 (and ½, if you are reasonable and use fractional saves), and you should put 4 ranks in Intimidate.
Precocious Apprentice is from Complete Arcane, in a sidebar on page 181 rather than in the feats chapter. Some argue that it cannot qualify for prestige classes that require “ability to cast 2nd-level arcane spells,” usually on the grounds that you have only one. But so does a Sorcerer 4, and no one’s denying them entry on those grounds.
Invisible Spell is from Cityscape. Really, any metamagic feat works here, I chose this one primarily because it doesn’t change the spell level. This means you can use it on your Precocious Apprentice spell.
Furthermore, Complete Arcane has this gem:
Sudden Metamagic Feats: These metamagic feats don’t require modified spell slots, and so they work as well with spell-like abilities or invocations as they do with spells
As far as the Sudden feats are concerned, this rule is solid. But Sudden feats are not the only metamagic feats that don’t require modified spell slots (for instance, Insivible Spell), and the wording here implies that this is a general truth about how metamagic works with spell-likes. RAW, this clause doesn’t have the authority to make that statement about the general case, and it’s the only reference in the rules to this idea. That makes it pretty invalid, strictly speaking. However, it is a pretty neat idea that helps out a relatively weak class, so consider allowing it. It would mean you could use Invisible Spell on invocations. Consider eldritch glaive for slicing people up with no apparent motion on your part.
You might alternatively consider Extend Spell for eventual Divine Metamagic shenanigans.
Warlock 1/Binder 1
This gets you 1st-level vestiges. It also bumps your base Will up to +4 (+5 if using fractional).
Warlock 1/Binder 1/Duskblade 1, Improved Binding
Duskblade has a good Fortitude save, bringing it up to +2 (or, if being reasonable and using fractional saves, +3⅙). Honestly, literally any good-Fort class would work here. I chose duskblade because it’s thematic, it gets Will too, arcane attunement is solid enough, and it has its own arcane caster level, which will be relevant in a bit.
Duskblade is from Player’s Handbook II.
Improved Binding means you qualify as a 3rd-level binder for vestiges’ sake, which means 2nd-level vestiges. Between this and Precocious Apprentice, you now qualify for...
Warlock 1/Binder 1/Duskblade 1/Anima Mage 3, Spell Focus (evil)
You have a 2nd-level arcane spell, you have 2nd-level vestiges, and you have “any metamagic feat” and 4 ranks in Intimidate. So you qualify for and can take levels in anima mage. Anima mage can progress warlock just fine, for all that warlock has some difficulty entering. Three levels brings you to 6th overall, with 4th-level warlock invoking and 4th-level binder pact-making.
Anima mage is from Tome of Magic.
You also have +9 as a base Will save (+11 if using fractional; less sane, but skipping the additional +2 each time you take a new class with a good save is typical and appropriate, even though Unearthed Arcana does not do this—that would give you a far more reasonable +5), and +3 base Fort save (+4⅓ with fractional). By taking Spell Focus (evil), you qualify for...
Warlock 1/Binder 1/Duskblade 1/Anima Mage 3/Ur-priest 2
Ur-priest is an exceptionally appropriate class here: it’s evil, it’s all about stealing power you shouldn’t have, and it gets 9th-level spells in as many levels. Two levels gets you 2nd-level divine spells and Rebuke Undead.
Ur-priest is from Complete Divine.
Warlock 1/Binder 1/Duskblade 1/Anima Mage 3/Ur-priest 2/Eldritch Disciple 8
Eldritch disciple advances invocations, and except at 1st, divine spellcasting. After 8 levels of it, you’ll cast spells as a 9th-level ur-priest, enough to get 9th-level spells.
You’ll also have the invocations of a 12th-level warlock, i.e. greater.
We could have done this with mystic theurge instead of eldritch disciple, saving us a level but costing us any and all class features. Eldritch disciple’s class features are mediocre, but they’re better than nothing, and things work out so that this doesn’t really cost us anything.
Eldritch disciple is from Complete Mage.
Warlock 1/Binder 1/Duskblade 1/Anima Mage 7/Ur-priest 2/Eldritch Disciple 8
Another four levels of anima mage bring binding up to 8th level, enough for two vestiges of up to 5th level (thanks to Improved Binding). It also brings warlock invoking to 16th, exactly what we need to hit the dark invocations.
We have 9th-level divine spells off the cleric spell list, dark invocations off the warlock list, and a pair of 5th-level vestiges off the binder list. We used the 1st, 3rd, and 6th level feats, as well as the human bonus feat, but the rest are available, for instance to grab Shape Soulmeld and Open Chakra feats, or by getting into Divine Metamagic.
With Flaws, and extreme cheese
Unearthed Arcana’s flaws allow a character to gain an additional two feats at 1st level, in exchange for said flaws. This is extremely useful here.
Anima mage is also amenable to extreme levels of early entry shenanigans. As in, 2nd level entry shenanigans.
Warlock 1/Anima Mage 10/Ur-priest 2/Mystic Theurge 7
The two feats from flaws can be Bind Vestige and Improved Bind Vestige from Tome of Magic. This gives us the ability to bind a single vestige, which grants only a single power, as a 5th-level binder, i.e. up to 3rd-level vestiges. That allows us to eliminate the level of binder altogether.
Thus, entry into Warlock 1/Anima Mage is entirely possible, and absurdly, it advances us as if we were taking binder levels even though we have none to begin with. Since we no longer have a level to burn at 3rd (we used to because we needed to wait until 3rd to get Improved Binding), we’ll skip duskblade and delay ur-priest until 9th or 10th (depending on the use of fractional).
We can, of course, take Improved Binding once we get into anima mage. If retraining or psychic reformation is available, our Warlock 1/Anima Mage 2 can take Improved Binding and safely ditch Bind Vestige and Improved Bind Vestige.
I also switch to mystic theurge because it allows us to complete anima mage entirely, so we get vestige casting, i.e. the ability to cast a spell as if Still and Silent and as an immediate action (!). Once per day, but who cares, that’s incredible. Eldritch disciple’s got nothing on that.
We end up with the invoking of an 18th-level warlock (dark invocations), the spellcasting of a 9th-level ur-priest (9th-level divine spells), and the pact-making of a 10th-level binder (a pair of 6th-level vestiges assuming you take Improved Binding).
Hitting 6th-level vestiges is a big deal. That means you get Zceryll, the Star Spawn, which is hands-down the most powerful vestige in print (assuming you allow it at all, which you might consider not doing). For whatever reason, the 6th level of vestiges is also oddly crowded, with nine entries (second only to 4th level, which has ten). There are only seven total vestiges that you don’t get, and only four of those are in Tome of Magic.
Including a bit of other things
We do still have four feats (six, in the second case and using psychic reformation to undo Bind Vestige and Improved Bind Vestige). That’s enough to do a few things.
If we are azurin instead of human, we still get a bonus feat, but instead of the bonus skill points we get a single point of essentia. Take Shape Soulmeld, Bonus Essentia, and some other [Incarnum] feat—they each give one essentia—and you’ll be able to shape one soulmeld and max out its capacity (which is based on character level). You also have room for at least one more feat, for Open Least Chakra, Open Lesser Chakra, or Open Greater Chakra, depending on where you want to bind this soulmeld.
If using flaws and psychic reformation, Expanded Soulmeld Capacity (or an incarnum focus) and another [Incarnum] feat lets you fill your souldmeld with five essentia. Getting to six is possible using Expanded Soulmeld Capacity and an incarnum focus, but you can only fill that through more psychic reformation cheese to get another [Incarnum] feat.
Tome of Battle maneuvers are inherently easy to tack onto any character. You get half your level to your Initiator Level, can take Martial Study up to three times, and Martial Stance as much as you want (but since you can only use one stance at a time, you wouldn’t want more than one anyway). That’s four feats, if you choose to use them that way.
On the other hand, your BAB will be atrocious, and most of the maneuvers of the Sublime Way involve attacks or improve attacks, so those are not especially useful to you. Still, there are some excellent options:
Shadow jaunt or shadow stride – teleportation is useful to anyone, and move-action teleportation, in the latter case, is actually unavailable to you from all your spellcasting. Shadow stride would only be available by using your 18th-level feat on Martial Study, however.
Distracting ember – swift-action is easy, and if you have a rogue in the party, free flanking is nice.
Mountain hammer – great for breaking into or out of places, since it can go through walls, doors, locks, and so on. Note however that objects tend to have a fair amount of HP as well as hardness, so this may still be slow-going unless you invest in some damage improvements.
Counter charge, Setting Sun throw maneuvers – these don’t require BAB too much, since they work like trip attacks that can use Strength or Dexterity. Depends a lot on whether or not you have much of those. Note that Small size can be an advantage with these, however.
Iron heart surge – completely remove anything... kinda. Talk to your DM about what exactly it can and cannot end. Do not use the Customer Service answer; it’s terrible.
White Raven tactics – completely broken if you can use it on yourself, and arguably overpowered even if you can’t. Still, it’s a great maneuver, and getting it through Martial Study eliminates most of the big problems.
Step of the wind, child of shadow, hunter’s sense, bolstering voice, leading the charge, absolute steel stance, dance of the spider, roots of the mountain, leaping dragon stance, shifting stance, step of the dancing moth, press the advantage – all stances that are useful to you. Pick one that you like best.