Class Level Summary: Warlock (35), Monk (1), Blackguard (3), Rogue (1)
Race: Halfling
Starting abilities: Str (13), Dex (16), Con (8), Wis (8), Int (14), Cha (16)

Ending abilities: Str (13), Dex (16), Con (8), Wis (8), Int (14), Cha (26)

Least: Frightful Blast (requirement for Lord of All Essences), Dark One's Own Luck (useful to cap important save, perhaps replace later when you got +20 from items)

Lesser: Brimstone Blast (must have), Eldritch Chain (I suppose there can be situations where it can be better than Doom)

Greater: Vitriolic Blast (must have), Chilling Tentacles (must have), Wall of Perilous Flame (must have), Caustic Mire

Dark: Eldritch Doom (must have), Utterdark Blast (must have), Dark Foresight (must have), Retributive Invisibility (must have), Binding Blast (very useful for NPCs with low will), Instill Vulnerability

Start with Hungry Darkness, this invocation is the only AoE that Warlock has early and it is just overpowered. It has no limit on number of targets and most NPCs don't have ultravision so they will stand clueless inside or trying to attack missing all the time. I recommend to do hunting+alchemy craft to craft your own ultravision potions then you don't need Devil's Sight invocation. Or just take it…. Leaps and Bounds is quite good early too as it is basically +2 AC.

Note that you need either Frightful Blast or Eldritch Chain to penetrate Crypt boss spell immunity. So if you don't have help from another player perhaps wait for level 6, when you get access to lesser invocations, before taking on him.

Once you get access to lesser invocations take Eldritch Chain and then Brimstone Blast. Brimstone Blast changes damage to fire which is the most common element that NPCs have vulnerability to (and also immunity/resistance so if you see lower damage, disable it). Note that Eldritch Spear is not so bad either. That 1d6 damage bonus is worth it. Until you get access to Cone/Doom I recommend to take this too and use Chain/Spear based on how many targets there are.

At level 11 you get access to Greater invocations, if you don't have Haste from item take Flee the Scene. If you do, take Chilling Tentacles - this is the most powerful and universal invocation in Warlock's repertoire. After that Wall of Perilous Flame and/or Eldritch Cone.

Starting the moment you get Cone/Doom you will want to use it on NPCs that have low saves instead of using Chain. Remove Cone once you acquire Doom.

Do not forget that Warlock can cast in light armor. So wear leather armor for AC. Vendor one if needed as it might be hard to find good random one.

Great at killing Balrog, including HC version. Also Shadow Crypt including HC of course. Should be able to handle Minotaur Maze too, Moria, Frosty Tower and Kuo Toa although whether it can do HC I am not so sure about. If so probably not very fast and efficient I am afraid. So not very universal build as it cannot beat Mechanical dungeon. Hence off-meta. But it is fun to play for sure!

I still have to playtest this in the field. It is possible it needs a penetration feats rather than raw blast damage I took.

I highly recommend Halfling race, it is +1AC and immunity to Chilling Tentacles. The skill points are enough, or rather I did not find need for UMD since Warlock can use anything with Sorc/Wiz/Bard use limitation without UMD.

I don't recommend to use shield. Even that 5% arcane failure is too much and happens way too often. The 1 AC doesn't make a lot of difference, while under effect of Divine Shield, together with IE, you should have enough AC for everything.

This is just the starting build and you will want to remake it once you farm any of these: Kobold race book, Hellfire Warlock, Acolyte of the Skin.