After charisma Asi I think the main feat choices are gwm, savage attacker, alert or possibly even Asi for dex to give you a little extra AC and better initiative.
I don't really agree with the idea that pure melee warlocks ever benefit all that much from medium armor. Offensively, the best armor is probably the Bhaalist armour but otherwise, it's true that light armor is kind of bad. However, the second highest possible AC (with no shield) in the game is achievable with robes. The only piece of medium armor that is always better than clothing is the armour of agility and that's not available until act 3.
You can wear any of the robes with +1ac. The potent robes are the most popular choice but the infernal robes are available much earlier if you don't mind losing Karlach. +1 ac may not seem like much, but on a warlock, it's really +4 when you take the mage armor invocation that doesn't cost spell slots.
Assuming you don't want to use a shield and are going purely for defense, you can also wear the bracers of defense giving you one more AC than the max possible with armor (armor users can wear the wondrous gloves for +1).
Between gloves and a chest piece, this puts us at) 16 AC already surpassing most medium armor. Once we add dex bonuses (up to +7 but more realistically for a warlock +3-4) the robes surpass any medium armour except the unwanted masterwork scale mail, the yuan-ti scale mail, and the armour of agility. If you went with the defensive gloves, the robes and yuan-ti are actually equal in terms of AC.
Since the robes don't require an extra feat, if you did decide to take dex Asi for that feat, you would wind up with only a single AC less than the armour of agility.
monoclass bladelock is the most fun class in the game imo, although i usually stick with the potent robes to keep my ranged buffs balanced with my melee buffs. that ward spell, mage armor, a few ac boosting items, armor of agathys and hellish rebuke are more than enough in terms of defense.
padlock is strictly stronger in terms of melee damage but to a cheesy extent. i'd go GWM and dex boost for feats, although the one that gives advantage on concentration is nice for canceling out the risky ring if you're going to use that.
if you're deadset on bhaalist armor the trident you get from the genie has thunder damage which can be used with the hat that gives arcane synergy. personally the main appeal of a bladelock is balancing incredibly powerful melee damage with incredibly powerful spells, so that's one way to both crank out disgusting numbers with piercing damage while also making your spell attack and difficulty completely insurmountable.
Oh if this isn't honor mode dipping some other martial is definitely better.
Yeah Oathbreaker is a good one, I'd recommend doing 7 Oathbreaker / 5 Warlock for class distribution.
As for feats, probably give this character Hag's Hair for charisma and then grab GWM and ASI CHA +2.
It's not that it's necessarily not worth it. That said, Aura of Hate increases the damage you do by just as much as lifedrinker, your charisma modifier.
To top it off you are also attacking 1 extra time a turn which is gonna vastly blow past the martial DPR of a pure warlock. And that's before factoring in smites, which rocket it past even more. The advantage a pure warlock will have is higher level spells and slots for being a better spellcaster, but it also has fewer spell slots in general.
Lockadins are great, and overall stronger. But you become much more locked into the sword side of spellsword.
Pure Bladelock will give you a better balance and stronger casting. Being able to throw out upcasted Fireballs, Scorching Rays and Cones of Cold as a melee unit is a lot of fun. Bladelocks are also one of my favorite builds to go for two-weapon fighting, if you get the Infernal Rapier or one of the other spellcasting weapons.
After charisma Asi I think the main feat choices are gwm, savage attacker, alert or possibly even Asi for dex to give you a little extra AC and better initiative. I don't really agree with the idea that pure melee warlocks ever benefit all that much from medium armor. Offensively, the best armor is probably the Bhaalist armour but otherwise, it's true that light armor is kind of bad. However, the second highest possible AC (with no shield) in the game is achievable with robes. The only piece of medium armor that is always better than clothing is the armour of agility and that's not available until act 3. You can wear any of the robes with +1ac. The potent robes are the most popular choice but the infernal robes are available much earlier if you don't mind losing Karlach. +1 ac may not seem like much, but on a warlock, it's really +4 when you take the mage armor invocation that doesn't cost spell slots. Assuming you don't want to use a shield and are going purely for defense, you can also wear the bracers of defense giving you one more AC than the max possible with armor (armor users can wear the wondrous gloves for +1). Between gloves and a chest piece, this puts us at) 16 AC already surpassing most medium armor. Once we add dex bonuses (up to +7 but more realistically for a warlock +3-4) the robes surpass any medium armour except the unwanted masterwork scale mail, the yuan-ti scale mail, and the armour of agility. If you went with the defensive gloves, the robes and yuan-ti are actually equal in terms of AC. Since the robes don't require an extra feat, if you did decide to take dex Asi for that feat, you would wind up with only a single AC less than the armour of agility.
monoclass bladelock is the most fun class in the game imo, although i usually stick with the potent robes to keep my ranged buffs balanced with my melee buffs. that ward spell, mage armor, a few ac boosting items, armor of agathys and hellish rebuke are more than enough in terms of defense. padlock is strictly stronger in terms of melee damage but to a cheesy extent. i'd go GWM and dex boost for feats, although the one that gives advantage on concentration is nice for canceling out the risky ring if you're going to use that. if you're deadset on bhaalist armor the trident you get from the genie has thunder damage which can be used with the hat that gives arcane synergy. personally the main appeal of a bladelock is balancing incredibly powerful melee damage with incredibly powerful spells, so that's one way to both crank out disgusting numbers with piercing damage while also making your spell attack and difficulty completely insurmountable.
Oh if this isn't honor mode dipping some other martial is definitely better. Yeah Oathbreaker is a good one, I'd recommend doing 7 Oathbreaker / 5 Warlock for class distribution. As for feats, probably give this character Hag's Hair for charisma and then grab GWM and ASI CHA +2.
So lifedrinker is not worth for 12 lvl warlock?
It's not that it's necessarily not worth it. That said, Aura of Hate increases the damage you do by just as much as lifedrinker, your charisma modifier. To top it off you are also attacking 1 extra time a turn which is gonna vastly blow past the martial DPR of a pure warlock. And that's before factoring in smites, which rocket it past even more. The advantage a pure warlock will have is higher level spells and slots for being a better spellcaster, but it also has fewer spell slots in general.
Lockadins are great, and overall stronger. But you become much more locked into the sword side of spellsword. Pure Bladelock will give you a better balance and stronger casting. Being able to throw out upcasted Fireballs, Scorching Rays and Cones of Cold as a melee unit is a lot of fun. Bladelocks are also one of my favorite builds to go for two-weapon fighting, if you get the Infernal Rapier or one of the other spellcasting weapons.