I'm working on an idea for a backup character in a campaign. 

The backstory's rather straightforward. Pirate's ship is sunk in a fight with other pirates. My character is drowning, but she's offered a second chance at life by a kracken, who will make a pact with her to save her life and grant her power, in exchange for one day being called upon to return said favor for the Kracken's own unknown goals. 

The character is a variant human and starting at level 12, since that's the level the main party in the campaign is at. Stats are point buy. 

The general idea I have in mind is a melee focused warlock, fighting in the thick of things with her tentacle, using it to both attack with her bonus action and sticking near it so that she can use its reaction to defend her from attacks and help mitigate damage, since she's not going to have a super high AC or anything. 

Taking a couple levels into rogue is tempting for cunning action, but considering attacking with the tentacle is a bonus action that would be competing with cunning action in the action economy. The character would also be wanting high CHA for the tentacle attacks and high DEX for AC and her own attacks with finesse weapons, and so it's probably not a good idea to multiclass and miss out an an ASI. Since she'll always be proficient with her pact weapon, will probably go with rapier for the pact weapon's form, or maybe cutlass if I want to play into the pirate theme a bit more and take a slight hit to damage. But definitely sticking to a finesse weapon since she'll only have light armor proficiency. 

Being able to cast the black tentacles spell without requiring concentration and getting temp HP means she'd likely favor this spell in combat as well, as she won't have to worry about losing concentration on it while melee fighting, at more risk of losing concentration becuase of it. Needing both high dex and cha means she probably won't take much in terms of feats instead of asis, but she ould take warcaster or resilient (con) potentially as her variant human feat for other spells. 

Thirsting blade seems pretty much mandatory for the second attack, any advice on other invocations to grab? 

Does anyone have any advice on making this work? I know that hexblade is usually the go to for a melee warlock, but I like the flavor of the fathomless and curious to see if this can be built to be, if most likely not optimal, at least decent?  ( I don't care about min maxing, but I also want to make sure the character is useful to the party and not a liability.) 

That sounds like a great concept! I like the idea that you're trying for a melee build without hexblade. I think the main thing you'd be missing is medium armour proficiency. Of course, if you're going for a swashbuckling type character then light armour may make more sense from an RP perspective. You could still consider the Moderately Armoured feat. It'd give you medium armour proficiency as well as an increase to Str or Dex.

I know you didn't want to multiclass, but you could consider looking at another class that gives you armour proficiency. In fact, you could even consider a single level in Barbarian. Seeing as how you're looking at a concentration-light build with that tentacle, rage wouldn't give you as many concentration issues. That tentacle can still be slapping away while you're raging. You'd also get unarmoured defence, which could help replace the light armour depending on your constitution.

I hadn't thought of monk or barbarian for that, I'd mostly been thnking 'warlocks get light armor so I'll go for a finesse weapon and use studded leather.' 

One clarification though. To be more specific, I do have to concentrate on the black tentacles spell. It's just that the concentration on that spell specifically can't be broken by damage. (Sleet storm etc could, but no having to take con saves to maintain it every time she's hit.) So she couldn't keep that going AND rage at the same time. 

Medium armor could work, but my thought was since I'll need good STR or DEX for weapon attacks anyway, I might as well go dex with studded leather. Though I could also go a single level of fighter or something for heavy armor as well and go strength... 

Another thing to ponder is the Mobile feat
"When you make a melee attack against a creature, you don't provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not."

If you want better armor, one level dips into Cleric are always nice.

A dip into nature or or tempest cleric I think would work. Probably nature because she's not going to have the WIS to make frequent use of tempest's level 1 feature much, but both would give her the option of going STR instead of DEX with heavy armor too. 

My thought is that if I take a half feat that gives +1 dex or +1 cha, on top of my two human asis to put into both, I can pot those stats at 14/15 in point buy and have both at 16. Then spend the 4/8th level asis to bump both to 18. If I go full warlock I'd get one more at 12, to either take for fun/flavor or bump one of the stats up to cap. And still have 14 con, which combined with using my reaction on guardian coil will hopefully be enough to keep her up through combat.  Or go cleric and swap out dex for STR in the above. 

I see, so barbarian wouldn't be a great multiclass option. I realize now it also requires 13 Str, which you may not want.

If you're looking at cleric, I'd like to suggest the Twilight and Forge domains. Overall, the domain's proficiencies may not be that useful since you may not have the Str to use heavy armour. However, the level 1 features of these two domains are still quite good.
Forge domain is basically a passive +1 weapon or armour. This can also be swapped between your armour and your weapon each long rest.
Twilight domain would give you 300ft darkvision that can be shared with some of your party. It also lets you give someone advantage on every initiative.
Trickster domain could also be helpful if you plan to use a breastplate, or if you plan to do a lot of stealth since has Twilight's advantage mechanic but for stealth rolls.

Honestly... I'd just stick with Pact of the Blade and stay a Warlock. If you have a decent Dex, Armor of Shadows has you covered.

However... you could always take a look at Paladin - while it's not the 'optimal,' Oath of the Open Sea is full of flavor, and gets you a lot of what you were looking for above, without being particularly MAD. It's 3 levels to make it worth your while, but...

Yeah I'm kind of leaning toward full warlock. Probalby capping my dex for max AC and weapon attacks, with cha at 18 for now and to be capped if the character ever gets high enough for another asi. She can live having her cha at just 18. 

Devil sight for darkvision/magical darkness seeing. In part for the flavor of her being able to see in the darkness of the sea if she ends up going there. Which considering the nature of her patron isn't unlikely. 

Armor of shadows for free mage armor, thirsting blade for two attacks, lifedrinker for stronger attacks. 

Not sure if improved pact weapon is worth it, there's a good chance the DM might just give her a simple magic weapon starting at level 12, so the +1 might be redundant. And with her dex based using a finesse weapon she's going to have a hand free to use her focus if she needs to without stopping her attacks to cast a spell. So I don't think she'd necessarily benefit from that aspect either. 

That still leaves her level 7 and 9 invocations free, maybe I should take agonizing blast for situations where enemies are out of melee reach. (She could carry a bow but if her pact weapon is a melee weapon, she can't attack twice with it so may as well EB.) Maybe Grasp of Hadar to help pull enemies closer as well to help keep enemies from getting out of range of her and her tentacle, which she can only move 30 feet a turn. Eldritch smite is always an option but as much as I enjoy playing my paladin when I get the chance, not sure I want to use my spell slots here on smiting too. 

Armor of Agathys could be another useful survivability boost on top of mage armor and the tentacle defensive reaction. 

Quote from Nyr_Ventus >>

Yeah I'm kind of leaning toward full warlock. Probalby capping my dex for max AC and weapon attacks, with cha at 18 for now and to be capped if the character ever gets high enough for another asi. She can live having her cha at just 18. 

 

Devil sight for darkvision/magical darkness seeing. In part for the flavor of her being able to see in the darkness of the sea if she ends up going there. Which considering the nature of her patron isn't unlikely. 

Armor of shadows for free mage armor, thirsting blade for two attacks, lifedrinker for stronger attacks. 

Not sure if improved pact weapon is worth it, there's a good chance the DM might just give her a simple magic weapon starting at level 12, so the +1 might be redundant. And with her dex based using a finesse weapon she's going to have a hand free to use her focus if she needs to without stopping her attacks to cast a spell. So I don't think she'd necessarily benefit from that aspect either. 

 

That still leaves her level 7 and 9 invocations free, maybe I should take agonizing blast for situations where enemies are out of melee reach. (She could carry a bow but if her pact weapon is a melee weapon, she can't attack twice with it so may as well EB.) Maybe Grasp of Hadar to help pull enemies closer as well to help keep enemies from getting out of range of her and her tentacle, which she can only move 30 feet a turn. Eldritch smite is always an option but as much as I enjoy playing my paladin when I get the chance, not sure I want to use my spell slots here on smiting too. 

Armor of Agathys could be another useful survivability boost on top of mage armor and the tentacle defensive reaction. 

 Yes to all of this...

And AoA is the way to go... because it's not JUST defense, especially when you upcast it. It scales beautifully...

(and I love the Grasp of Hadar to pull back into the tentacle - so many ways to skin that...)

Here's how I'd do it:

Start off with 15 DEX, CHA, & CON. Put your v.human +1s into CHA and CON, get the feat moderately armored and +1 dex.  Then, for 4th lvl ASI I'd take warcaster. I really wouldn't push my luck as a frontliner relying on mage armor alone, especially if I'm concentrating on something. So, with half plate and a shield I'd be at a comfortable 19 AC while keeping my spell casting and fortifying my concentration, plus, that number can most definitely increase if you find +1 shields/armors or improve in  other ways like with adamantine armor. 

Now, here's where things would branch off and I'd give a hard think about: how you attack. If you go and max DEX from here on out, your damage with lifedrinker and attacking twice with thirsting blade would be (1d8+5 from DEX +3 from lifedrinker)*2, or 25 damage a round.  By contrast, let's say you use booming blade instead. Even if the target never moves away on their turn they would take 1d8 from the rapier, 2d8 from booming blade's initial hit, 5 from DEX, and 3 from lifedrinker, or about 21.5 damage per round. However, keep in mind you could potentially bump that up to 35 damage per round if you can proc BB's secondary hit. Did I mention BB and warcaster oppie synergy? 

Basically, you have the option to attack twice per round, or attack with booming blade for similar damage. The obvious upside is that you wouldn't need to worry about using an invocation for thirsting blade if you decide to go with BB as your primary means of attacking. 

Then, I'd go for improved pact weapon. Even if you may be given a +1 weapon, the fact you can use your weapon as a focus means material components are covered even if your hands are full with a rapier and shield. Keep in mind, this +1 applies to magic weapons that lack a +1 to begin with, so you'd be boosting things like a flame-tongue rapier, dancing sword, or frost brand. Instead of asking the DM to start off with a +1 weapon, you could ask for +1 armor or shield.

Personally though, I'd go and max out CHA. or 18 CHA and 18 DEX, keeping warcaster in mind. You're still a warlock with spells and all, so I'd want to keep those DCs up especially if you're going to be using spells like black tentacles, while relying on the +1 from the improved pact weapon to mitigate the lesser to-hit bonus. 

Level 12 as a variant human gives you 1 feat and 3 feat/asis.

I quite like cgarciao's build idea. Certainly starting with the moderately armored feat allows you to start with three 16 stats (and three 8s) and medium armour and shields.

And Warcaster, and 18 for dex and charisma seems like the best options to me. Use Improved Pact Weapon to summon a +1 rapier so you can fight using dexterity while holding your shield.

I play one they work fine

For full warlock 12

• Max out Dex and Cha and use finesse weapons with studded leather or mage armor
• For warlock spells take Armor of Agathys, Hellish Rebuke, and Hex
• You will have to make a choice between casting Hex or Evard’s black tentacles from grasping tentacles as they both use concentration. Id lean more towards Evards because there is a level of enemy control and damage wont break your concentrate on it (and its free to cast) but both are fine.
• Now you are a "hit and run" melee combatant.
  • Stay half your movement away from the front line of melee combat. Move in and attack using booming blade if the creature is alone or thirsting blade second attack if you have an ally that can give you Flanking advantage and make opportunity attacks against the creature if they try to chase you or if you think you can kill the creature with one hit.
  • Bonus Action attack with the Tentacle of the Deeps.
• After you attack retreat back the other 1/2 of your movement HOPEFULLY triggering Opportunity Attacks.
  • Melee warlocks are kind of like masochists and do a lot of damage by getting hit. Hellish Rebuke and Armor of Agathys the opponent if they opportunity attack hits. You want them to hit you so you can destroy them so don't stress about getting that AC too high either. For this kind of combat things like the Mobility Feat wont help.
  • If they try to follow you booming blade will trigger as well.
• Repeat this In and Out style of combat until you run out of spell slots and/or temporary hit points from Armor of Agathys.
• Guardian Coil is going to help you stay in melee this way. Keep it within 10 ft of where you will be attacking. Use it to reduce the damage from those opportunity attacks and giving you some temp hp (don't forget temp hp don't stack so be mindful to not cancel out your Armor of Agathys)
• Once you are out of Temp HP and/or Spell Slots retreat as far as you can and get behind as much cover as you can and act like a Eldritch Blast Spam warlock for the remainder of the fight. 

Another Option for fun is Cleric 2 Warlock 10

You could also take 2 Lev in Tempest Cleric. In that case Max out STR and CHA (keep WIZ at 13) but basically its the same but with heavy armor and the ability to max damage thunder and lightning spells. In this case definitely take Lightning Bolt for that sweet sweet MAX DAMAGE Lightning Bold damage. And for cleric spells take ones where you Wisdom bonus doesn't matter much (like guidance, thaumaturgy, bless, protection from evil and good, shield of faith). Use the tempest Wrath of the Storm as soon as you can. Its not a high save with 13 wisdom but still does damage even if they make the save.

Aside form the spells mentioned above you are free to take whatever other spells and invocations you like for utility and fun. Eldritch Smite is killer. Darkness and Devils Sight is a good choice. Lots of utility options to help out your group. Sculptor of Flesh and Cloak of Flies are worth considering for both combat and out of combat utility. 

Enjoy!