Hexblade

A Hexblade is an arcane/martial hybrid class in Dungeons & Dragons.

3rd Edition D&D

In 3e he is more or less an arcane Paladin, just slightly worse.

He has curses, full base attack bonus progression (+1 per level sweet fuck), a companion familiar (instead of a mount), has good will saves, casts magic spontaneously, has a big hit die and gets the occasional bonus feat dedicated to improving spellcasting.

The class is superficially like a Duskblade, which is also a hybrid Arcane/Martial class. While he learns to cast spells later and only casts fewer spells-per-day, his spell list is improved, making him slightly more variable, but unfortunately that's where the comparison ends.

As a caster, he cannot wear anything bigger than light armor, his other saves suck dick and cannot be good (becoming so results in the loss of all your powers and your familiar). Also, his class features don't entirely mesh well with his function, since cursing and arcane resistance is all well and good, his spellcasting abilities do not make him a replacement for the groups primary arcane caster and without any combat features he doesn't stand up front as a fighter either. So the Hexblade doesn't really work as either a Caster or a Warrior. Duskblades do this much better.

He will also need a combination of Charisma (for his spells), Strength (for his attacks), Dexterity (to not die) and Constitution (also to not die), leading to heavy MAD. Pathfinder would later port the hexblade as an archetype of the magus class, called a hexcrafter. They sacrifice their spell recall ability to gain access to curses in their spell list and the ability to learn witch hexes instead of magus arcana.

Making Hexblades Work

You will be best served by taking spells that increase your own fighting abilities and survivability through not getting hit: by picking your spells wisely you can fill up a few niches as they up, from flanking an enemy and serving as the rear guard to punishing enemy casters. Do note that the class is not considered to be very good, because while the Hexblade can fill certain openings as they fall he cannot fill them as good as someone fit for that job can.

Prestige class options are very similar to the Duskblade options, made slightly easier to qualify later in your career since you have a broader range of spells. Unfortunately it can't really be said which way you "should" go, since you picked a character class that didn't really have a job to start with.

Dragon Disciple from the core rules works for the Hexblade moreso than Bards or Sorcerers who have to give up much of their spellcasting potential in order to gain buffs that take them out of their comfort zones. As a "Fighting class" who can spontaneously cast, sacrificing most of your spellcasting ability (no great loss) for a series of permanent physical buffs that could help steer your character into a front line role, rather than the confused position the Hexblade normally occupies. Though unlike the Prestige class options noted below, the BAB of the Dragon Disciple follows the 3/4 progression of the cleric, so you do trade away some accuracy for the added strength (which net-outweighs the BAB loss unless you're looking for an extra attack early on) defense and class features, so it's your call...

Abjurant Champion from Complete Mage works acceptably, since it improves your combat abilities by allowing you to burn unused spell slots for temporary bonuses. Other features improve your survivability, but this is mostly situational since your caster list does not include Mage Armor as standard, so you'll have to settle by augmenting your other protection spells, such as Shield.

If you are Elf/Half-Elf, the Bladesinger class works alright with Hexblade levels, though it means fencing yourself into a duellist role by fighting one-handed and makes you more MAD considering that it needs INT to function to its best. Just don't take it too far, as you start losing out on class features as you can already cast in light armor.

Spellsword is where you want to be if you want to stick it as a Hexblade and improve your combat abilities, though you need to have both Medium/Heavy armor proficiency and level 2 spells, meaning you need be around level 8+ to take this class.

If you can pick up the Rage ability from somewhere, either by multi-classing one level of Barbarian (or taking the Eye of Gruumsh prestige class if you are Half-Orc/Orc) then Rage Mage might be the class for you, while it improves neither your spellcasting or your BAB to its best possible ability, the class features do synergize well with the Hexblade, allowing you to cast spells while raging and wear medium armor without penalty.

4th Edition D&D

In 4e they work differently. Like a Warlock a Hexblade has forged a pact with powerful entities like the Fey or Fiends, but instead of getting PHENOMINAL COSMIC POWAH he gets a sword instead. Most of the time the Hexblade's patron gives this so that it can be used to shed blood/souls of the unbelievers for the glory of the blood god Hexblade's patron, who might be the blood god. They use their sword in one hand and an implement in the other, making for an offense-oriented character. As above, they require a combination of Charisma, Strength, Dexterity and Constitution to work, making them still MAD across editions.