I am trying to work on a number of character builds as options for playing Call of the Netherdeep when it comes out. I have a lot of ideas but am having trouble getting them to work mechanically, so I wanted to open up discussions here about them to kind of brainstorm these builds with everyone here so I can put them together so that I have them ready for when the adventure book is released. Here I would like to discuss my idea for what I call the ‘Eldritch Wizard’ (which doesn’t actually mean taking levels in Wizard, though it is an option, just not a great one.)

So let’s go over my idea, thematically as well as mechanically.

Thematic concept:
So this is a character that is a bookworm and has spent most of his teenage and young adult life studying in the city library. He has come across some magic tomes and through reading them has taught himself how to cast some spells. Eventually, he comes across or acquires an ancient tome of forbidden eldritch magic and immediately gets entranced by it and takes it away into his private study to unlock its powers. Unbeknownst to him, the book is a communication device that an otherworldly entity uses to communicate to potential servants in the material plane. He starts having weird dreams and even nightmares before the entity approaches him in his dream and they make a pact.

Mechanical Concept:
So this is a multiclass character that mains in Warlock but has levels in some other full spellcasting class (Wizard, Cleric, Bard, Druid, or Sorcerer.) He is a blaster mage, making full use of the Eldritch Blast cantrip, as well anyone blasting spell. The recent of his spells are control and debuffing spells. However, he learns all his warlock spells from his ancient tome, though he is able to learn and write down other spells that are not given to him by his patron.

So as far as multiclassing goes I wanted to pick a secondary class that was good with books and had spells that he can prepare at the beginning of a day. The only options that I saw were Scribe Wizard, Knowledge Cleric, or Lore Bard, but each of these has problems. The Wizard and Cleric would make him multiple ability score depended, and bard and cleric spells don’t really line up for a blaster

Also, to fit into the world of Exandria (specifically the Wastes of Xhorhas of Wildemount, I was thinking of making him either:

Dark elf, Half-Dark elf, custom lineage Dark Elf (from the city of Rosahna, the capitol of the Krynn dynasty);

Dragonborn, or custom lineage Dragonborn (from the city of Xartzith Kitril)

I was considering custom lineage because it would allow me to pick up Magic Initiate, which might ease up the burden of having to dip into another class at level 1 first, allowing my character to gain some spells of another class while he is doing his initial leveling through Warlock. One more thing of note, because this is going to be a potential character for Call of the Netherdeep, this build needs to start at level 3 (the starting level of the adventure).

Here are a couple very different suggestions:

• Most effective yet still thematic Secondary Class - Aberrant Mind Sorcerer.
• Weirdly thematic but very useful Secondary Class - Bladesinger Wizard.

I'd go Sorc and just flavor your weirdo sorc powers as also coming from your patron, or as changes to you from your contact with them, or something. Then just Pact of the Tome for the book learning stuff alone. Take the telekinetic feat. Shoots beams at people, hex em, shove em around and read their minds. Good warlock vibes. And cast ritual spells from your tome.

Another lock multiclass option is to go bladesinger and not worry about too high a Int, 13/14 is enough. I know, weird. But hear me out: Your goal here is to get to Wiz 6 to pick up the bladesingers ability to cast a cantrip (EB obviously) while also making attack actions. This works best if your patron is a hexblade, though. So, ymmv if you're going a diff warlock subclass than that one. Focus on picking up all te ritual spells your heart can desire as well as utility spells and whatnot, then your blasting comes from hex and then both attacking and blasting at the same time, both using your cha mod.

Happy hunting!

You don't need to multiclass to get the ability to scribe spells into a spell book as a warlock... If you take the book of ancient secrets invocation it gives you a the ability to add ritual spells to your pact time. Sure, it's only ritual spells, but if you're focusing primarily on being a blaster in combat this gives you some good utility spells that you can use without dipping into your spells lots.

Why do you want to multiclass?  What's the goal/point?

Just to illustrate that it can be done, here's a Cleric/Warlock build, because you mentioned it as a possibility.  But I don't understand what you're trying to do, so I can't guarantee it'll be satisfying. 

Tasha's Half-Elf Life Cleric 1 / Celestial Warlock 2; Future Plans: Become a Chainlock at L3 and stay a Chainlock forever.  Note that for this particular build, class order doesn't matter.
Starting Stats (Point Buy): 15 Cha / 14 Con / 14 Dex / 12 Wis / 8 Str / 8 Int
Post-Racial Stat Mods:      17 Cha / 15 Con / 14 Dex / 13 Wis / 8 Str / 8 Int; Future Plans: At level 5 take Elven Accuracy (Charisma), at level 9 take +2 Cha, at level 13 take Resilient Constitution.  Wisdom can stay at 13, the build is Charisma-based.
Invocations Known:            Agonising Blast, Repelling Blast; Future Plans: Chain Pact at L3 and learn Gift of the Ever-living Ones, Devil's Sight, and Eldritch Mind post-haste.
Warlock Cantrips (4):        Light, Sacred Flame, Eldritch Blast, Create Bonfire
Cleric Cantrips (3):        Guidance, Mending, Thaumaturgy
Warlock Spells Known (3):    Hex, Hellish Rebuke, Armor of Agathys
Cleric Spells Prepared:        Bless and Cure Wounds are auto-prepared.  You have 2 more discretionary spells prepared.  Protection from Good and Evil is an example of a good preparation choice, but you do you.
Note on gear:  This build basically assumes Medium Armor, a Shield, and a Pew-Pew Eldritch Laser (i.e. an empty hand).  You can re-spec it for Heavy Armor (you *are* proficient), but you'll slow down unless you find a stat point for Strength.

 Note that to fit the theme, you'll need to ask your DM to consent to flavoring your Pact of the Chain Familiar as being shaped like a book.

You can re-do this as a Celestial Tomelock (at which point Cleric becomes less useful since you're doubling up on sources of Ritual Casting).  But I don't know why you want to multiclass, so it's hard to make good build choices on your behalf.  It would be unbelievably easy to drop Cleric entirely and use 1-3 levels of Divine Soul Sorcerer - I'm sure you've at least heard of Sorlocks.