An unseelie fey warlock: intense, compelling, mysterious. Ever-changing, always-dangerous. Some warlocks focus on eldritch blast and damage, and have little use for Charisma, but I don’t think that’s you—I imagine you’ll be taking few Eldritch Essences or Blast Shapes, and focusing a lot on invocations and other effects that increase the number of ways you can mess with your opponents.

Distinctly Fey Stuff

This is stuff that involves being a fey, or having fey heritage, or interacting with fey. The reasons these are appropriate should be obvious.

Racial Stuff

The unseelie fey template from Dragon vol. 304 is extremely potent for its minimal level adjustment. Probably unreasonably so, actually; many DMs will balk at it. Then again, the warlock class is on the weak end so maybe your DM will allow it.

Pixie warlocks are somewhat traditional: their permanent greater invisbility is much better than the walk unseen invocation (which is as the spell invisibility and therefore broken by offensive actions), and their racial flight saves you an invocation on fell flight. That said, LA +4 hurts, to the point that I don’t really consider the pixie playable.

Another good fey is the petal from Monster Manual III: they’re Tiny, which makes them damnably hard to hit, and eldritch blast doesn’t care what size you are. They also get Improved Initiative as a bonus feat, and large ability score bonuses. They are “LA +2 (cohort),” which is frequently taken to mean that they cannot be player characters but only cohorts (e.g. from Leadership). However, a quick check at least reveals no actual rules for that entry, and really there’s not really any good reasons why petals cannot be player characters. Many DMs allow it, if forum posts are anything to go by. Note that LA +2 is still very problematic in general; I probably wouldn't do it unless I was starting at 8th level (and therefore had access to the warlock’s walk unseen invocation).

For both the pixie and the petal, you’ll have to do a fair bit to darken the character since those are, by default, fairly innocuous and pleasant fey. The warlock class is pretty good at that. The dead walk and summon swarm would be great choices for that. They’re pretty good choices in any event.

There are only two LA +0 Fey-type races I can think of, however: the killoren of Races of the Wild and the dusklings of Magic of Incarnum. The killoren are very “protectors of nature”-y, which I don’t think fits, though they can get Smite, which works well with your Charisma focus. Dusklings, on the other hand, are pretty fitting (Small size, midnight-blue skin, somewhat crazed appearance), but the −2 Intelligence hurts you because you’re going to want skill points and you only have 2+Int skills. That said, if you do go that route, Magic of Incarnum actually has some really good invocations in it, though the best one (incarnum blast) doesn’t need any Essentia to do its thing (since the Daze effect is way more important than the bonus damage).

Feats

Nymph’s Kiss (Book of Exalted Deeds) is a great feat for anyone, and warlocks can put it to particularly good use with their in-class Use Magic Device. It grants +2 on all Charisma-based skill and ability checks, a bonus skill point per level, and fey treat you as fey for the purpose of Diplomacy and negotiation. Also, you get a faerie lover (who is good-aligned, which helps since non-good fey can get quite ... jealous). The bonus skill point is particularly important since you want good skills, especially social skills, and have only 2+Int skills from warlock.

The Faerie Mysteries Initiate feat from Dragon vol. 319 is ridiculously powerful, though less so for a warlock than it would be for, say, a wizard. One of the options (Passions) is the ability to use Intelligence instead of Constitution for determining your bonus HP. Note that this is another feat that grants you benefits for getting it on with a fey. The biggest reason you would take Faerie Mysteries Initiate is so you can dump Constitution safely, and be rewarded more for pumping Intelligence, which in turn lets you get the skill points you want to be the beguiling silver-tongued trickster you really ought to be. Though the warlock’s 2+Int skills will constantly make that difficult.

Wizards also ran a regular column on their site called Fey Features; Life in a Noble House is one of the better ones. Several feats:

• Beauty’s Bounty, which is a much-tamer, Cha-based version of Faerie Mysteries Initiate. Ultimately not great but decent.
• Charmed Life, which is similar to the granted power of the Luck Domain.
• Charming the Arrow, which allows you to use Charisma instead of Dexterity on bow or crossbow attacks (you might be able to convince your DM to allow this to apply to eldritch blast, even though it explicitly doesn’t—that would be a great feat).

As well as Un/Seelie Court Member/Noble Kelir, which are both pretty lame mechanically, but the flavor in the article may be useful for describing your character, even if you don’t take the feats (which I don’t recommend you do). The Fey Kelir Magics article may also give some good ideas along the same lines.

Other Unseelie-ish Stuff

These are options available to you, as a warlock, that I think will enhance your unseelie flavor. I’m focusing on the creepy and the changeable here: fey are capricious, after all.

Races

The only LA +0 races with bonuses to Charisma are hellbred (Fiendish Codex II) and spellscales (Races of the Dragon), both of which take penalties to Constitution, which hurts a lot. Better if you can do Faerie Mysteries Initiate, though. There is also the so-called “lesser” aasimar, which also gets +2 Wisdom and is frankly overpowered, and not very fey, particularly unseelie. Actually, none of these are, I’m just mentioning them because you do care quite a bit about Charisma.

Alternatively, the changeling race (Eberron Campaign Setting) is like a match made in heaven here. They’re master tricksters, with the natural ability to change their appearance at will, their very name comes from fey folklore (a changeling is a fey creature substituted for a human child and left to be raised by the human parents). You can also aim for the excellent chameleon prestige class (which I’ll describe below), since Able Learner requires human or doppelganger and changelings are half-human, half-doppelganger hybrids. Also consider dipping rogue; a changeling rogue has a lot going for it (see below).

Other Classes

Dread Witch

The dread witch prestige class would be pretty solid, but it’s impossible for the warlock to enter. See if your DM will allow you to enter using frightful blast.

Binder

You probably aren’t going for hellfire warlock (though you should consider it; there’s no reason why an unseelie fey cannot have deals with devils), but even without it, a single level of binder (Tome of Magic) works really nicely: it gives you abilities you can change frequently, which is appropriate for capricious fey, and Naberius in particular gives you stunning social abilities. And Naberius is excellent for hellfire, if you go that route.

Cleric

While we’re talking about dipping in other classes, cleric is never a bad choice, between Turn/Rebuke Undead, Domains, and a smattering of spells. Cloistered cleric would also be a great 1st level, since it has 6+Int skill points. Knowledge Domain can also be traded for Knowledge Devotion (Complete Champion), which is a great boost for eldritch blast. Cleric also opens up entry to eldritch disciple (Complete Mage). Warlocks are pretty good at counterspelling, and with a cleric dip you can take Divine Defiance (Fiendish Codex II) to counterspell as an immediate action. To me, a fey-themed warlock should be about twisting his enemies about more than just straight blasting them; hard to argue with that kind of action denial.

Changeling Rogue

If you are a changeling, taking your first level as rogue (before starting warlock or anything else) has a ton to recommend it with the racial substitution levels from Races of Eberron. First of all, 10+Int skill points. Which, at 1st level, is ×4—that’s huge. Second, Social Intuition is awesome. For the record, Sneak Attack does apply to eldritch blast. For a non-changeling, starting with rogue is less great, though still worthwhile. I’d probably favor cloistered cleric, though, even over the changeling rogue.

Chameleon

Finally, the chameleon prestige class (Races of Destiny or here) is awesome, and two levels of it serve a warlock very well. Again, I’m focusing on fey being capricious: chameleons can change their abilities every day, including a feat they can change every day that can be a different Extra Invocation as you need it. Particularly potent if you don’t take the dead walk so that you can use your floating feat to get it when you need it: you do not lose control of your undead if you change your Extra Invocation to something else after you use the dead walk to make them. That’s pretty cool and very appropriate, I think, for this character. Plus, if you get to warlock 12/chameleon 2 you can make any magic item in the game by swapping in the requisite Item Creation feat and using Imbue Item.

Invocation Choices

You want a lot of lesser invocations, I think. If you are not a fey that has its own flight and/or invisibility, fell flight and walk unseen are obvious choices. Frankly, they’re almost every warlock’s choices for lesser invocation. And I’ve already mentioned why I think the dead walk is great for you.

And then there’s charm. I can’t imagine more fey-appropriate invocation. And it’s really quite good, though [Mind-Affecting] and [Language-Dependant] are going to limit its applicability badly.

Summon swarm is one of the best low-level invocations, and fits an unseelie fey for obvious reasons: death, decay, but nature and magic. One of the few things warlocks are better at than their true-spellcaster cousins: their version of summon swarm ends as soon as they stop concentrating on it, which prevents friendly fire. Eventually, many warlocks will want to switch summon swarm for something else (beguiling influence or dark one’s own luck are common choices) since the damage and save DCs stop being relevant, though you may want to keep the ability for thematic reasons.

Other good low-level, appropriate choices could be baleful utterance (destruction, particularly of things like locks; seems fey to me), beguiling influence, dark one’s own luck, and see the unseen.

At the greater level, chilling tentacles and nightmares made real are excellent crowd-control that have a heavy “that which goes bump in the night” feel. Meanwhile, devil’s whispers, despite the name, could not possibly scream unseelie louder if it tried: as suggestion, except your target then must make another Will save at a −5 penalty or forget they did what you suggested. Seriously, the chaos you can create with that in the right situation is astonishing.

For dark invocations, dark foresight seems quite appropriate, given fey’s association with omens, while word of changing is at-will baleful polymorph, which shows up in... almost every myth the fey have ever made an appearance in?

You probably won’t focus on your eldritch blast as much; you certainly have and use it, but you’ll probably want more variety in your invocations than simply getting new attachments for your gun. That said, if you want some essences, frightful blast and noxious blast seem fitting and appropriate, not to mention they’re simply quite good. If you do focus on eldritch blast damage (hellfire warlock, say), vitriolic blast becomes basically required to make sure that damage gets through SR.

I’d also mention that Complete Arcane claims that the various “Sudden” metamagic feats work on invocations “because” they apply no spell level adjustment. RAW, this is dubious as anything—the actual wording of those feats offers nothing that would allow them to apply to spell-like abilities like invocations, and the lack of spell level adjustment is completely irrelevant to anything as far as the rules for metamagic feats are concerned, but, if you buy that line and allow metamagic without spell level adjustment to apply to invocations, I would mention the excellent Invisible Spell metamagic from Cityscape. An invisible eldritch glaive is one of my favorite mental images in all of D&D 3.5e.

Some Inspiration

Elves are wonderful. They provoke wonder.

Elves are marvelous. They cause marvels.

Elves are fantastic. They create fantasies.

Elves are glamorous. They project glamour.

Elves are enchanting. They weave enchantment.

Elves are terrific. They beget terror.

The thing about words is that meanings can twist just like a snake, and if you want to find snakes look for them behind words that have changed their meaning. No one ever said elves are nice.

Elves are bad.

—Lords and Ladies by Terry Pratchett

It's a shame it's too late to make class suggestions as this character's ripe for factotum (Du 14-20). More levels of wizard would really be a boon, too. She's already a wizard, so she's a conjuration specialist who took the alternative class feature abrupt jaunt (PH2 70), right? And since you don't care about her being especially effective, taking Mnk1 and beating up a sparring dummy of the master (AE 137) (30,000 gp; 40 lbs.) to get that 10 ft. of movement required for to use skirmish is kind of a thing. Anyway.

Obscure Feats

• The uncategorized feat Hardened Criminal (City of Stormreach 95) grants the creature immunity to attempts to Intimidate it and the ability to take 10 on 1 skill picked when the feat's picked. Note: This feat sprang to mind immediately when you mentioned your character's focus on the Escape Artist skill.
• The fighter, general, and Chult and wild dwarf regional feat Disentangler (Rac 162) grants the creature a +2 bonus to Escape Artist skill checks and a +2 bonus to opposed grapple checks.
• The general feat Daredevil Athlete (CS 76) as an immediate action 3/day grants the creature a +5 competence bonus to a single Balance, Climb, Escape Artist, Jump, Ride, Swim, or Tumble check. I assume the intent is the next one, though. As a competence bonus, it'll stack with the circumstance bonus from masterwork tools but not the competence bonus that often comes from magic items, which is sad.

• The aberrant feat Mourning Mutate (Dragon #359 110) as one of its benefits grants the creature a +3 racial bonus to Escape Artist skill checks from being "unusually flexible." It's 1st-level only and can make the creature look scary, awesome, or scary awesome at the DM's discretion. As it counts as the feat Aberration Blood (LoM 178) (which could instead grant the creature +4 racial bonus to Escape Artist skill checks, but the creature then has "slimy skin"--ew), it grants access to...

  • The aberrant feat Inhuman Vision (LoM 180) grants the creature darkvision 5 ft. per aberrant feat and a +1 bonus to Spot skill checks per aberrant feat.
  • The aberrant feat Scavenging Gullet (LoM 181) grants the creature a +4 racial bonus versus ingested poisons and diseases. More importantly, it grants the creature the ability to "gain nourishment from eating any organic material, despite its freshness or source." For a sensate, this is gold.
• The fighter and general feat Master of Mockery (Dragon #333 88) grants the creature the ability to make a Perform (comedy) skill check as a standard action. The check's target makes a Will saving throw (DC = the Perform (comedy) skill check's result). If the target fails, it's enraged, gaining a +2 bonus to attack rolls versus the creature, taking a -2 penalty to Armor Class, and attaking the creature "whenever able." Talk to the DM about the effect's duration.
• The tactical feat Combat Panache (PH2 93-4) grants the creature use of 3 tactical maneuvers that are wildly subpar or difficult to use but a a lot of fun. Two maneuvers actually require the creature to be hit and take damage before they can be used, and the other requires the creature to hit and inflict damage before it can be used. Nonetheless, they're flashy and fun.