Do you want to play a 
Warlock who loves his craft, deals cyclopean amounts of damage, and shows his enemies the true meaning of cosmic horror? If the answer is yes, then this build is for you.
Build Summary
This build is centered around the idea of maximizing the damage output of Eldritch Blast, the Warlock-specific cantrip that starts off as a simple 1d10 ranged spell attack, but later scales with your Charisma modifier and gains more beams, for a total of 3 on level 10. With 20 Charisma and some specific gear, a level 10 Eldritch Blast will deal 3d10 + 30 damage for the cost of a single action.
Each of the Eldritch Blast beams can crit, and so we’ll be doing everything in our power to extend our critical hit range. By mid-game, we’ll have a critical range of 17-20, and in the late-game, we’ll be able to boost it all the way to 14-20. And that’s not counting the 
Spell Sniper feat — now that it’s been patched to actually work, it adds another easy +1 to our crit range. All the critical hits we’ll be doing synergize really well with 
The Great Old One subclass that, among other things, allows you to Frighten your targets in an area of effect whenever you crit.
Character Creation
Here’s all you need to gaze into the abyss just long enough for it to gaze back and grant you a fraction of its power. In other words, here’s how you’ll be creating your level 1 Warlock.
- Race/Subrace – Wood Half-Elf
- Starting Class – Warlock
- Starting Subclass – The Great Old One
- Abilities
- 8 Strength
- 16 Dexterity (15+1)
- 14 Constitution
- 10 Intelligence
- 8 Wisdom
- 17 Charisma (15+2)
- Background – Charlatan
- Starting Cantrips
- Starting Spells
- Skill Proficiencies
- Deception (from Charlatan)
- Sleight of Hand (from Charlatan)
- Stealth (from Race)
- Intimidation
- Investigation
We choose the Wood Half-Elf subrace primarily for the Shield Proficiency. Stealth is also a good Skill to have for us, while extra movement speed and resistance to charm and sleep are nice bonuses for everyone.
We pick The Great Old One patron for the crit synergy, but later on, it will grant us some extra survivability through Entropic Ward and resistance to Psychic damage.
Some of our crit-expanding capabilities will come from hiding and staying in the shadows, as such, our Skill and Ability choices make us better at that, while the Charlatan background allows us to become a substitute 
Rogue of sorts. You won’t be as good as a dedicated Rogue at picking locks or pockets, but you’ll be able to do it in a pinch. It’s also completely viable to grab a single level of Rogue at some point without losing too much in the process.
The only cantrip we’ll really need is Eldritch Blast, so we grab Mage Hand along with it for the situational utility it provides. And as spells go, Hex offers great synergy with Eldritch Blast throughout the game, and Dissonant Whispers is a nice way to apply the Frightened condition before you can start critting reliably.
Spells and Abilities
This section is dedicated to all the new abilities you’ll be picking up along the way on your journey to greatness.
Feats
There are two mandatory choices you’ll want to make as your number one and number two Feat picks:
A fairly important part of this build is to raise your Charisma to 20 as soon as possible. An easy way to do it is to make a deal with Auntie Ethel in Act 1. Since making deals with dangerous otherworldly entities fits the class theme, this choice will not only make you stronger, it will provide you with an opportunity for some neat roleplaying.
If you don’t get it as your first feat, grab Spell Sniper at level 8; besides that, your third Feat (if you go pure Warlock all the way to level 12) is pretty much up to you. Some viable choices include another Ability Score increase to boost your Dexterity or Constitution, and Alert to avoid ambushes and act faster in combat.
Should you decide to not make a deal with Auntie Ethel, you might also consider grabbing Actor for the extra Charisma and greatly improved Deception checks.
Pact
None of the Warlock pacts are really all that great, but for this particular build two of them are borderline useless. We won’t be attacking with our weapon, so Pact of the Blade does nothing for us. And none of the Pact of the Tome spells work well together with our kit.
As such, we’ll be picking Pact of the Chain. The familiar it grants isn’t very strong, but it’s still another body on the field, which can come in handy at times.
Eldritch Invocations
There are some really helpful Eldritch Invocations for this build. These will be marked by an *. The others are merely suggestions and can be freely swapped for something else.
- Level 2
- Agonizing Blast*
- Repelling Blast*
- Level 5
- Level 7
- Level 9
- Level 12
Spells
This build won’t be casting a lot of spells that aren’t Eldritch Blast. The only two spells that are bordering on mandatory are Hex and Darkness, a level 1 and level 2 spells respectively. The rest are entirely up to you, but we generally recommend focusing on utility spells like Detect Thoughts, Misty Step, Dimension Door, and Telekinesis.
Equipment
This build requires a great deal of specific equipment to unlock its full potential. Great news is, you can find most of it by early Act 2, with certain Act 3 pieces taking it to the next level.
Some things you might want to keep in mind is that you don’t need to have a weapon in your hands for it to grant its bonuses. So, even though the build lists some bows we’ll be using, we won’t ever use them to shoot, just have them in our ranged weapon slot as “stat sticks.” Another sticking point is the fact we won’t have Proficiency with some of our weapons. But since, again, we won’t be using them to attack, that doesn’t bother us in the slightest.
With that out of the way, here’s all the gear you’ll need to make this build work:
- Knife of the Undermountain King
- A shortsword that increases our critical hit range by 1. You get it from A’jak’nir Jeera in Crèche Y’llek.
- Bow of the Banshee
- A bow that grants us extra 1d4 to our Attack Rolls against Frightened targets. It should also grant us extra 1d4 damage, but that property seems to only work for physical attacks. Not clear whether that’s a bug or not, but additional 1d4 to hit is nothing to scoff at either way. You can loot it from Corsair Greymon on your way to Grymforge.
- The Dead Shot
- A late-game replacement for the Banshee bow. Increases our critical hit range by 1. You can get it from Fytz the Firecracker in Lower City.
- Potent Robe
- The main piece of gear for this build. A robe that adds our Charisma bonus to our cantrip damage (which stacks with the normal Eldritch Blast damage bonus), increases our AC by 1, and grants some bonus HP every turn. You get it as a reward for completing the Rescue the Tieflings quest in Act 2.
- Covert Cowl
- A hat that increases our critical hit range by 1 while obscured. You can loot it from one of the Meenlocks in the Last Light Inn basement.
- Risky Ring
- A ring that grants us Advantage on Attack Rolls and Disadvantage on Saving Throws. The glass cannon option. Can get it from Araj Oblodra in Moonrise Towers.
- Disintegrating Night Walkers
- Boots that let us cast Misty Step and make us immune to slipping and Enwebbed, Entangled, or Ensnared. You can loot these from Nere the True Soul in Grymforge.
- Daredevil Gloves
- A pair of gloves that grant us a +1 bonus to our spell Attack Rolls and let us ignore the disadvantages of casting spells when threatened by enemies. You get them from A’jak’nir Jeera in Crèche Y’llek.
- Ketheric’s Shield
- A shield that grants a +1 bonus to Spell Save DC and spell attack rolls. As you can imagine, you get it from Ketheric Thorm at the end of Act 2.
- Shade-Slayer Cloak
- A cloak that increases our critical hit range by 1 while hiding. You can buy it in Act 3 when visiting the Thieves’ Guild.
- Bloodthirst
- The definition of a late-game item, you get it by defeating Orin in Act 3. It increases our critical hit range by 1, and grants us +1 AC when wielded in off-hand instead of a shield.
- Elixir of Viciousness
- An elixir that increases our critical hit range by 1. You get plenty of ingredients to craft a lot of these in Act 2.
The Gameplan
The basic idea of this build is to hit things with Eldritch Blast. However, to maximize our potential damage, we’ll want to utilize some tricks.
In order to get good use out of the Shade-Slayer Cloak, we’ll want to start fights in Stealth. While to make the Covert Cowl work, we’ll want to stick to obscured parts of the map. You can tell that a spot will obscure you if you press Shift and your cursor will show an empty circle instead of the usual pointer. But generally, if you see a large shadow on the ground, that’s where you want to be. Once you’re obscured when wearing the cowl, your character will get a buff with a spider icon, so you can always tell if you’re doing it right.
Alternatively, since you’ll have Devil’s Sight, you can just cast Darkness on the ground, run into the cloud and attack from there. Not only will the magical darkness obscure you, it will grant you Advantage on attack rolls even without the Risky Ring and prevent most enemies from attacking you back. As such, Hex and Darkness will be your two primary non-Eldritch Blast spells throughout the game.
Love the build! Very flavorful! I found that I was enjoying Pact of the Tome more than Pact of the Chain. While the familiar is nice to have at times, it didn’t feel spectacularly impactful. In places where I needed to get through a pipe or a hole, Mage Hand was enough. Pact of the Tome isn’t terribly powerful, but if you’re running without Shadowheart in your party, Tome gives you access to Guidance, and after Deepened Pact at Level 5, you’ll be able to cast Haste! If you manage to time things right with Haste and stealth and start the battle with both, oh boy are you about to ruin some enemy’s day.
If anyone is reading this and wondering what spells to take aside from the ones listed in the guide, I would recommend the following Spells/Cantrips as you level up, focused on utility since this build prioritizes Eldritch Blast:
1 – C: Mage Hand, Eldritch Blast; S: Dissonant Whispers, Hex
2 – S: Armour of Agathys. Armor scales per level, so it works great with Warlocks!
3 – S: Darkness
4 – C: Friends; S: Invisibility. Don’t ever take Charm Person, Friends does the same thing in dialogue.
5 – S: Grant Flight. You don’t need Misty Step if you’re using the Disintegrating Nightwalkers
6 – S: Hunger of Hadar
7 – S: Slow
8 – S: Dimension Door
9 – S: Hold Person
10 – S: Remove Curse
11 – S: Gaseous Form
12 – S: Hold Monster
Doesn’t seem necessary to get terribly high in warlock… Could take 3 fighter levels to boost the crit rate 1 more. There’s also a super late game helmet that doesn’t require hiding to boost the crit rate (Horned helmet). By my count, if all of that, along with spell sniper, one day works, you’d be able to get a crit range boost of +8:
-Champ
-Spell Sniper
-short sword
-dagger
-bow
-Cloak
-Helmet
-Potion
Alongside the normal crit range (i.e. a 20), that would be a 9/20 of 45% chance. Combined with advantage, that’s a [1-0.55^2] = ~70% chance at a crit on any given hit.
With all that in mind, it almost seems like a waste to use it just to scare people… Love the build, and the effort that went into the research though!
The fear is there mostly to add some RP flavor to a build that’s already strong enough.
And good catch on the Champion levels. In our tests the only multiclass that added some value to the build (withouth diluting what it’s about so it becomes hey – you can do all this, but also be an almost pure Sorcerer) was a level of Rogue that makes the character a great damage dealer/face/scout all rolled into one.
That said, something like Warlock 8/Rogue 1/Champion 3 would indeed be even better and allow us to eventually wear the Horned helmet you mention. The level progression would look something like Warlock 2 – Rogue 1 – Fighter 3, then continue leveling Warlock. Edit: upon further consideration – this progression is terrible, as that delays Feats too much and you need them both (provided sniper gets fixed). So W4 – R1 – W4 – F3 or W2 – R1 – W6 – F3 would be better, depending on whether you rush the Potent Robe or not.
But if you’re looking to just play a Warlock that’s simple, really strong, and has a theme – nothing wrong with going pure Warlock all the way.
Warlock level 8? Why not go for sorc as well for quickened spell? So you can cast 3 eldritch blasts in the first turn.
If you go Sorc, you want to maximize the Sorc stuff, so that comes up to something like Warlock 2/Sorc 10. And then you’re just playing a Sorc with a splash of Warlock. And you have all these spells you want to cast instead of spamming EB. I see it as a bit of a waste. That is not to say it’s not a strong combo.
And only investing 3 levels to get quicken for 1 extra blast per long rest – that’s not very impactful. If you need even MOAR blasts, better to just get Quickspell Gloves for the same effect that can be used per short rest.
3 sorc levels gets you more than 1 extra blast per long rest, though, because you can turn your warlock spell slots into sorcery points and then refresh them each short rest. 9/3 Warlock/Sorc would get you 2x 5th level warlock spell slots to play with. Even sacrificing only one of those spell slots per short rest would get you 15 more sorcery points, adding up to 6 extra quickened casts of eldritch blast per long rest (more, if you have a bard in the party). Or you could get to Sorc 5, hasten yourself each combat and just have more casts that way.