How about this?
I looked at the Lich class, removed quite a bit, added some Warlock things to it, and came up with the below. Note that I consider Paralyzing Touch, Fear Aura, etc invocation equivalents, so they are not gained (nor is the damaging touch, nor are the ability increases, etc). Equivalent invocations can be created for the character to take for those abilities, if the character wishes.
The loss of Con and the d4 HD should help balance this, as should the high cost in xp and gp every other level. I almost also reduced the BAB to low. Casters typically get low, but undead typically get medium. You are not undead yet, but your prior caster class experience - the Warlock - also gains medium also, so . . . I eventually decided to keep it at medium.
I left the name blank [ooo] for now. Somehow Warlock-Lich seems too silly.
ooo
Pre-Req: ??? Needs Imbue Item (Warlock 12th level ability). Anything else needed? Maybe ranks in Know (arcane)?
HD: d4.
BAB: medium.
Proficiencies: none gained.
Saves: Poor Fortitude, Poor Reflex, Good Will.
Skill Pts: 2+Int (Technically undead gain 4+int, but 1) that would be a bit strong considering how much else is gained these levels, and 2) you are not undead until the final level. Unlike hp, skill pt increase is not retroactive.)
Skills: Bluff, Concentration, Craft, Disguise, Intimidate, Jump, Hide, Knowledge (arcane, the planes, religion), Listen, Move Silently, Profession, Search, Sense Motive, Spellcraft, and Spot. (Warlock skills + Lich racial skills)
01 .Imbue Phylactery I (Con -2), DR 3 / magic, Hidden Mind +2, Natural Armor 1
02* DR 3 / magic + cold iron, Light Fortification, Warlock Resistances +3
03* Imbue Phylactery II (Con -4), DR 5 / magic + cold iron, Eldritch Blast +1d6, Hidden Mind +4, Natural Armor 2
04* DR 7 / magic + cold iron, Medium Fortification, Warlock Resistances +5
05 .Undead Transformation, DR 10 / magic + cold iron, Heavy Fortification, Hidden Mind +8, Natural Armor 4
* Invocations: The ooo counts levels 2, 3, and 4 as warlock levels for the purpose of determining the number of invocations known and the highest rank of invocations that can be learned. A Warlock 12 / ooo 3, for instance, knows 8 invocations. A Warlock 14 / ooo 4, however, knows 10 invocations and gains the ability to learn Dark invocations at Warlock 14 / ooo 3
Imbue Phylactery: The ooo gains the ability to use Imbue Item to create a Phylactery. The item chosen must be magical (and thus also a masterwork), and it must have either a hollow, a gem, or a mind (ie: an intelligent item). The ooo must pay 1/4 the cost of the phylactery (30,000 gp; 1,200 xp). Immediately upon doing so the ooo suffers permanent Con loss of -2.
At first level it is presumed that the ooo will be creating the phylactery as soon as he has the means. Thus, he cannot progress a level (whether chosen of this PrC or not) until he has paid this cost.
Note also that while hp, bab, saves, and skill points are gained upon taking this level, no specials (DR, Hidden Mind, Natural Armor) are gained until the potential phylactery is imbued.
At level 3 the ooo must again imbue the Phylactery, costing another 30k gp and 1,200 xp. Note that, again, no specials for the level are gained until this cost is paid. And, again, the ooo permanently loses -2 Con (for a total of -4 Con).
At level 5 the ooo finally completes his phylactery. As before, no specials are gained for the level until this cost (double the prior costs) is paid (60k gp, 2,400 xp). Upon this final infusion, the ooo becomes undead, as per Undead Transformation, below.
Damage Reduction: At level 1 the ooo gains DR 3 / magic. When attacked, this counts separately from his normal DR 2+ / cold iron.
At level 2 these combine: the ooo now has DR 3 / Cold Iron and Magic. At level 3 this increases by +2 to DR 5 / Cold Iron and Magic. At level 7 it again increases by +2 to DR 7 / Cold Iron and Magic. Finally, at level 5 these increase by +3 for a final total of DR 10 / Cold Iron and Magic.
Eldritch Blast: At level 3 the ooo's Eldritch Blast is increased in strength by +1d6.
Fortification: As more of his body's functions come to rely on magic, as more of his soul and mind are bound to the phylactery, so too does his body become less affected by attacks that would slay a normal mortal. The ooo gains Light Fortification at level 2, Medium Fortification at level 4, and Heavy Fortification at level 5. Note that the Heavy Fortification is not gained until the final imbuement of the Phylactery is completed.
Hidden Mind: The ooo gains a +2 bonus to saves vs Mind Affecting Effects due to his mind being partially bond to his phylactery at level 1. At level 3 this increases to +4. Finally, at level 5 - due to his transformation into an undead, it becomes +8. As he still has a mind, he is not entirely immune to MAEs.
(Note, the +8 vs MAEs for aware undead (instead of true immunity) is a common house rule among those I play with. If your DM approves, you could take the core lich view and have true immunity. I think that might be a bit much, considering this is only 5 levels, but perhaps I am wrong in this.)
Natural Armor: Upon imbuing the potential phylactery, the ooo gains a +1 increase to natural armor upon any he might already possess. This increases by a further +1 at level 3 (again, after the potential phylactery is further imbued), and by +2 (for a final total of +4) at level 5 (again, after the creation of the phylactery is completed).
Warlock Resistances: The ooo gains an increase of +5 to his chosen energy resistances at level 2 and again at level 4, for a total of +10 to his chosen energy resistances at level 4 (a total of 15, due to the resistance 5 the ooo gained at Warlock level 10).
Undead Transformation: At level 5, after the final imbuement of his newly complete phylactery, the ooo becomes Undead, gaining all the benefits and penalties thereof, as described below, as well as the specials listed for level 5.
+ Darkvision 60 ft.
+ Does not need to breathe, eat, drink, or sleep.
+ HD increase to d12s for all levels (Rather than re-rolling all his Hit Dice, just add 4 hit points for every d4 he previously had, 3 hp for every d6, 2 hp for every d8, and 1 hp for every d10.)
+ Immune to damage to its physical ability scores (Strength and Dexterity).
+ Immunity to death effects, disease, fatigue, exhaustion, paralysis, poison, sleep effects, stunning, and any (other) effect that requires a Fortitude save (unless the effect also works upon objects).
+ Not subject to Critical Hits, Death from Massive Damage, Non-leathal damage, ability drain, or energy drain.
+ Uses Cha mod for Concentration checks.
- Can be Turned and Rebuked.
- Cannot heal damage naturally (via rest, etc). Must receive magical healing to restore hp.
- Con score is reduced to --, causing complete loss of Con based extra hp, Con bonus to Fortitude save, etc.
- Is immediately destroyed if its hit points are reduced to 0 or less.
- Not affected by raise dead and reincarnate spells or abilities. (Resurrection and true resurrection can affect undead creatures. These spells turn undead creatures back into the living creatures they were before becoming undead.)
- Positive energy (such as Cure spells) now harm, while Negative energy (inflict spells, etc) now cure.
The Phylactery
Unlike most undead, the ooo has a means of avoiding true destruction should their hp fall to zero. Instead, the ooo reappears 1d10 days later next to their phylactery (often appearing grasping it or otherwise holding or wearing it, if possible). The only true way to destroy the ooo is to find and destroy the ooo's phylactery.
As previously described, the phylactery is often a hollow object, a gem, or an intelligent object (self-aware in some manner, although arguably the ooo's mind replaces the former self-aware nature upon the completion of the phylactery at level 5). The object gains a bonus to its hardness equal to half the ooo's total character level, and its hp is likewise increased by twice the total character level of the ooo. The Break DC of the phylactery is equal to 20 + the ooo's total character level. Finally, the caster level of the phylactery is determined by the total caster level (as a Warlock) of the ooo. A Warlock 12 / ooo 5, for instance, would have a caster level of 15.
Thus, if a Warlock 12 / ooo 5 (total character level 17) was using, say, a dagger (light blade: hardness 10, 2 hp) as a phylactery, it would have hardness 18, 36 hp, and a break DC of 37. Note, however that this presumes the Dagger is +0 (ie: non-magical). If it has any type of enhancement beyond this, the numerical enhancement would apply as normal. If the dagger were +2, for instance, the numbers would alter to: hardness 22, 56 hp, and a Break DC of 37 (note that this last is unchanged).
If the Phylactery is destroyed, the ooo does not lose any of its specials, but unless another is created (120,000 gp, 4,800 xp) prior to the next time the ooo reaches 0 hp, it will be irrevocably destroyed.
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On the one hand you have a hit to HD, Con, progression of invocations and eldritch blast, and a loss of the specials that would have been gained had you continued as a Warlock. I almost put BAB at low, for that matter.
On the other hand, you gain quite a few bonuses - especially at the final level, and a couple of which increase Warlock traits (blast +1d6, energy resistances)
All in all, the class feels a little bit strong to me - even having removed the lich racial bonus to some skills, the ability increases a lich usually receives, the damaging touch, and reducing several bonuses the lich typically receives (immunity instead of merely resistance to energies and MAEs), and spreading it all out over 5 levels instead of 4.
I think it is at the high end of balanced, power-wise, although I am uncertain enough to request it be attacked by balance masters viewing this thread.
The main factors that help balance it are the low hp (d4 and increasingly reduced Con), the low level of the benefits compared to the 12-17th level of the character, the high costs in gp and xp every other level - with the benefits of the level held back until the costs are paid, the fact that most notable benefits do not come until the end - by which point the character is at least level 17, and the fact that the class only grants resistance rather than immunities to MAEs and two energies. Not needing to breathe is nice, but at level 17 your foes are tossing around Wishes and Miracles, so it sort of balances out, I think. While turning is usually a minor concern, the fact that the character has no turning resistance will no doubt make it at least a possible concern. Especially considering that if the character is destroyed they cannot be raised or reincarnated (and resurrection will restore them in a non-undead state - perhaps at ooo level 4?, considering the level loss?). A feat from Libris Mortis, I think, grants Turn Resistance +4 (or is it +3?), so for the cost of a feat you can partially alleviate this potential problem.