I was thinking about this too, so here's my build (added an additional Feat from your houserules):
Lvl 4 Tiefling, Archfey Tome Warlock
This build points to forgive the melee and AC options for a more focused caster. I liked your idea of an hybrid, but seems to me that it really needs to be an hexblade, going in the direction of the other classical builds around. Tiefling adds some interesting features, while allowing to reach 20 CHA on lvl 4, increasing the overall effectiveness and chance to hit of the few spells you can cast.
By Lvl 4 you have:
- Cantrips: Eldritch Blast, Minor Illusion, Vicious Mockery, Thorn Whip, Shocking Grasp, Thaumaturgy, Friends
- Lvl 1 Spells: Hex, Sleep, Faerie Fire
- Lvl 2 Spells: Hold Person, Phantasmal Force
- Hellish Rebuke 1\day (2d10)
- Fire Resistance
- Agonizing and Repelling Blast
I chose Archfey over Old One for several reasons:
- Spells: Dissonant Whispers and Tasha's seems kinda redundant to me, especially when you can get Hold Person at lvl 4. Sleep it's an
excellent spell vs. low level mass enemies, it has no concentration
and grows with spell slots. Faerie Fire can grant advantage to the
entire party vs one or more hard targets. Both pacts have Phantasmal
Force which is a great disable for low int enemies and fun to use too.
- Pact Powers: Fey Presence is an area disable which fits more the theme than Telepathy granted by Old One. At lvl6 Entropic ward is
fine, but Misty Escape is simply awesome.
Invocations: I'd go to PimpMyBlast just because this is your only reliable source of damage in a lot of situations where CC or forced movement are not options - If all CCs fails, if you want to blast that minion in the backrows not worthy a spell, in long fights, ecc..
But if you like, you can forgo one of the blast invocations to get rituals, which can give you a bit more flexibility and a Familiar.
Feats: i'd really like going first at all costs with this build, so i'll take Alert. A well placed Sleep or Hold person the first round is no joke. Otherwise Moderately Armored can give you better ac with a shield, but going melee will require War Caster too to be a viable option.
Ability scores in order of importance: Max Cha, High Dex, High Con, Mid Wis, Low Int, Low Str
Role: cc\secondary caster
Playstyle: Disable high treat enemies as soon as you can. Use forced movement (Blast, Thorn Whip) to generate AO from allies and reposition enemies at your vantage. Minor Illusion to modify battlefield (covers, false gaps, ecc), Vicious Mockery to help ally, Shocking Grasp to disengage, Thaumaturgy\Friends as social tool.
Progression: At lvl5 you can get the infamous Devil's Sight + Darkness combo which can transform you into a walking nightmare of disabling madness, very hard to target. Lots of tactics here, disabling sight with plain movement is a powerful option, plus you attack with advantage; just take care of LOS of your allies. If you want more utility you can always get Rituals. Darkness is 1/day from Tiefling so you can also get a lvl 3 spell. While patron's spells are not that great, at lvl 3 you have 2 strong area disables (Fear and Hypnotic Pattern) and Counterspell, which is really useful if you are the only caster in the party.
Here is Lvl detailed progression:
lvl 1
Tiefling: +2 Cha, +1 Int, Thaumaturgy
Patron Fey: 1-turn Area disable
Spells: 0 - Eldritch Blast, Minor Illusion; 1 - Hex, Sleep
lvl 2
Invocations: Agonizing + Repelling
Spells: 1 - Faerie Fire
lvl 3
Pact of the Tome: Vicious Mockery, Thorn Whip, Shocking Grasp
Spells: 2 - Phantasmal Force
Tiefling: Hellish Rebuke 1/day
lvl 4
Spells: 0 - Friends; 2 - Hold Person
Feats: Alert (from houserule 4th lvl)
2 levels of Warlock are enough for full Eldritch Blast damage
You are absolutely right, there is not much reason to go above level 2 with Warlock from an optimization standpoint, if you only want Eldritch Blast.
You get every benefit from Eldritch Blast, without giving up much in return, since cantrips scale with your character level, not your class level. You will have to consider however that Eldritch Blast requires Charisma, and that one is the usual dump stat of an Eldritch Knight. So you will either be quite bad at blasting, or sword swinging. Or both. There is no synergy in stats, playstyles, preferred distance, equipment, nothing.
It is great for others
As every DnD edition with character options, 5E rewards specialization. You either excel at standing in the back rows and sniping enemies, or be a front liner. There is not much middle ground.
For a Sorcerer or Bard, 2 levels of Warlock is golden, to have a really strong cantrip when they run out of real spells. But not for a Fighter.
So can you take 2 levels of Warlock, for full Eldritch Blast progression? Yes
Is Eldritch Blast worth it from an optimization standpoint for a Fighter? No
Other parts of Warlock might work better for you
However, if you do not focus on EB, Warlock can be a really good addition.
- Hex improves your DPR quite a bit, considering how many times a Fighter can attack on higher levels. Temporary hit points on a kill are also nice (Dark One's Blessing)
- 2 levels of Warlock double the number of slots per day, and you can get improved darkvision, even more hit points, or free Detect Magic (Invocations)
- 3 levels can give you quite a lot of great spells not belonging to the abjuration or evocation schools. Invisibility, Mirror Image, Phantasmal Force or Misty Step, all excellent for a Fighter
- An invisible scout (Pact of Chain) or rituals (Pact of Tome)
- 4 levels give you an ASI
- 5 levels can give you Fireball, Stinking Cloud, Blink, Fear, Counterspell, or Vampiric Touch and an invocation
- 6 levels can give you a teleport+invisibility, Temporary Hit Points, or Advantage on an attack depending on your patron
I would go like this:
- 1 Fighter level (saving throw/armor proficiencies)
- 2 Warlock levels (2 spell slots, 2 invocations, patron feature)
- 5 more Fighter levels (Extra Attack, ASI)
- 3 more Warlock levels (3rd level spells, 1 invocation, ASI, pact)
- Remaining levels in Fighter
Best Answer
I love the concept, Dr. Strange was one of my first super heroes to read a comic about. Would love to know how you intend to do the Eye of Agamotto.
I would suggest Warlock for first level; then dive into Sorcerer for a level for the AC boost. Then back to Warlock for another 2 levels. Finally, finish up with Sorcerer. So it winds up being WSWWS+. That first level will scream by pretty quick (one, maybe two sessions). You do delay 2nd level spell slots along with invocations by one level this way, so that's something to consider. Of the choices presented for order of classes, I'd say go with WWWS+. Using a feat to get eldritch blast when you know you'll get it for free in a level feels like a wasted feat usage to me. Plus it leaves that first level feat free for magic initiate (see answer to 3).
Dissonant Whispers I think is the better choice damage wise. It starts at 3d6 and will scale to 11d6 if you cast it using a 9th level sorcerer spell slot. I personally would say that outweighs the disadvantage that Vicious Mockery forces on a failed save. Plus, you can get this at first level, as Vicious Mockery can't be taken until 3rd level using your tome. Another option would be to get both spells, as the cantrip may have more opportunities for usage, being at-will versus limited by spell slots.
Take Magic Initiate as your first level feat and choose cleric as the class to get the cantrips, and first level spell from. Take sacred flame this way, gaining your radiant damage at first level. Also, you could pick shield of faith as your first level spell to be similar thematically to the shield discs Dr. Strange summons for protection. Another benefit of Shield of Faith is that it has no increase for higher spell slots, and the 1st level spell gained by MI will always be cast as if from a 1st level slot.