u/Consistent_Metal3498

Pure Undead warlock (No multiclass) lvl 5
Wooden Half-elf (35 movement)
8 10 10 str wis int
17 Cha (+1 from elf accuracy bonus lvl1 feat hb rule) (+2 lvl 4 asi) so its 20
16 dex 14 con
Pact of the Tome - Shillelagh, Guidance, gfb
Invocations - DevilSight, book of ancient secrets (Find familiar, identify), agonizing blast
Cantrips - EB, Boomingblade, Magic arm
Main spells - Hunger of hadar, Summon undead, Agathys, Phantom steed, Darkness

How do you advice to build this character further keeping in mind it will likely to battle against fiends mainly.
Since the beggining of the campaign i struggled a lot in the fights with this character and was almost killed 3 times. The group consists of archfey warlock, trickster rogue and swarm ranger PCs. We are also accompanied by a lore npc cavalier fighter who i always give phantom steed, as she is the tankiest member of this group (My pc comes in second place and often ends up in close quarters melee fights)

Wanted to share a great build i seen at the table - good at all game stages, a bit weaker in early game because pam barb is king early game

Giant founding origin - ice giant strike to freeze an enemy in place, counters flying and only melee enemies. Works well story and mechanics wise with our future barbarian subclass!

Custom lineage barbarian

+GWM feat because it somehow works with thrown heavy weapons by raw

Pointbuy
16 str (+2)
14 14 dex con
13 wis
8 8 int cha

Skills and tools - i recommend athletics, perception and tinker tools as we will probably be the party mule and can make money from repairing weapons and armors from slain enemies. Every other skills is up to your imagination

lvl 1-6 Straight path of the giant barbarian - Dont rush headfirst mindlessly in a batle! Use throwing axes and spears and switch to greatsword when all hell breaks loose! All characters die easily at first 3 lvls, even barbarians! Also its important to say it first - reckless attack sadly doesnt work with throwing, so use other ways to attack with an advantage like allied controll and buff spells!

Choose sharpshooter feat at lvl4 for no disadvantage at maximum distance of our thrown weapons and ignore cover! -5 +10 part doesnt work sadly although nobody would take -10 penalty to attack rolls anyways

Aim to purchase scale mail, med kits and a cart with a mule to carry our goods at this stage

lvl 7-9 Multiclass into battlemaster fighter -

We are here for 3 main reasons - battlestyle, action surge and maneuvers

At lvl 7 take greatweapon figting style - As our main weapon is 3d6 thanks to an elemental cleaver feature, it adds average +2 to damage rolls which is the same as throwing weapon style. This feature will scale even further if you find any magic greatweapon with bonus dices like flametongue. Also it works if you get caught into melee combat.
You also get second wind, which is not a huge deal but can keep as going in longer days with many short rests.

At lvl 8 We get action surge. We will mainly use this for throwing more attacks into important targets. Personally i wouldnt use it first turn every battle to throw more attacks if you dont have advantage on them. Save it to completely nuke somebody with 4 advantage attacks at (3d6r<2(avg 12.5)+2Rage+3Str+10Ggm)x4+1d8(avg 4.5quicktoss) - 114.5 damage without magic items, so unless your gm hates you, you will do even more! We can 1turn kill archmage if we get close, hoorah!

At lvl 9 we get our fighter subclass - which will be battlemaster.
We want Quick toss maneuver to have a reliable bonus action attack nearly every turn, use it when you didnt drop somebody to 0 hitpoints or critted in a turn to not waste resources as we already sometimes can attack with gwm.
Also take precision attack for those close to enemy ac rolls! Third option is up to your taste!

Aim to buy - halfplate, scale mail or any metal adamantine armor because enemies will crit you often because of reckless attack! Dont care about +Ac bonuses, enemies in future will have such giant +to hits and advantage it becomes nearly impossible to not hit you, so protect against crits instead!

Magic items to snipe if avaivable -

Any belt of the giant strenght! Works great with our origin and subclass lore and mechanics, if not avaivable then atleast try for gauntlets of ogre power! As a multiclass we arent getting lvl20 barbarian feature for +4 to max str, so aim to cover this lack with items!

Any magic greatweapon, preferrably greatsword or maul as they work best with gwf style! Best options - normal +to hit and damage, flametongue, сorpse/giant/dragon slayer depending on your enemies and avaivability! Giant and dragon slayers can be tied well with your origin! Crystall blade is good too!

Items with +to our saving throws! - stone of good luck, ring and/or cloak of protection - mental saving throws are our main weakness!

lvl 10-12 Multiclass into Gloomstalker ranger -

We want thrown weapon fighting style, expertise in athletics and some helpfull spells!

Lvl 10 - We get +1d4 to our weapon attacks against certain target with free action! Hoorah!
Also an expertise in athletics, as we are large when we are raging and have giant +8+str as a modificator with an reliable advantage with rage, we can knock prone or grab nearly anything! At this point you have an upperhand with athetics even if you are fighting stormgiants! Grabbing and knocking prone same target means it cant get up at all because it has 0 movement speed, use it wisely!

Lvl 11 - We are now a spellcaster! Choose absorb elements for instances when you get obliterated with magic or breath attacks before you could rage! Use longstrider as it is non concentration hour buff to our movement which we will try to use often! Positioning is important!
We also get thrown weapon fighting style, +2 to our thrown weapon, good!

Lvl 12 - We get our third subclass! Gloomstalker ranger gives as honorable +1 to initiative boost at this point, which will become +2 in future!

Our main goal is here! Dread ambusher allows as to make 3 attacks with our action in the first turn! This works well with action surge, so in your first turn you can obliterate anyone who relies on dark vision from 60 feet with 7 attacks!

Assuming we atleast get +1 magic weapon at this point and somehow rage instantly before initiative roll we could deal
(3d6r<2(avg 12.5)+1magicweapon+2Thrownstyle+2rage+10gwm+1d4rangerfeature(avg 2.5+5str(Assuming we could get atleast hillgiant belt!))x7+(1d8Dreadambush(avg 4.5))x2+1d8(avg 4.5)quicktoss+1d6(avg 3.5)Icegiantstrike= 248 mixed damage! Assuming this all landed with reliable +5 with advantage, you just downed Adult black dragon and his wyrmling kid in the first round! Brutal!

You can also grab zephyr strike for situations where you are running! Good i guess?

This build core is completed at lvl 12 and scales later nearly purely from items. I would recommend dumping all remaining lvls into barbarian so we can eventually become 3x3 huge giant at lvl 20!
Take resilient wisdom for nasty controll spells and now +2 to initiative from dreadambush feature! With remaining last feat i would recomend taking something situational like ritual caster, mobile, lucky or Fury of the frost giant for flavor!

Thats all folks, good luck and enjoy trowing greatsword 7 times in a first turn at a flying dragon to completely overkill it!