The Duskblade is a character class for 3.5 Edition D&D that can be found in the Players Handbook II.
The class itself is very similar to the Hexblade character class; being a martial class with access to INT based, spontaneous arcane spellcasting. The big difference is that it came after Wizards of the Coast realized being able to cast arcane spells in armor wasn't actually all that powerful and classes with it didn't need to be hit with a nerf bat repeatedly. The Pathfinder equivalent is the magus, and the Hexblade is an archetype of the same.
It beats out the Hexblade in that it has better saves, earlier access to spells that continue on to a higher level, learns how to cast spells while wearing up to medium armour and the Duskblade is not bound to a non-good alignment. The Hexblade has better hit dice and has curses that are a pain in the ass to fight against, but doesn't get that much at higher levels.
The Duskblade spell list is quite limited, restricted to only damage dealing or buff spells. Which is perfectly fine since they're the fighter, not the dedicated caster. Though when you get to the higher levels, you get to be able to cast a phenomenal 10 spells per day of 1st-3rd level spells, maxes out at 6 5th level spells, and that makes Duskblade the only class that Ring of Wizardry is worth the cost for.
Given the fact that a duskblade can channel damage-dealing spells directly through melee attacks, they slightly surpass dedicated fighters in most melee situations (which isn't hard, because non-casting dedicated fighters tend to suck). The only builds in which fighters will surpass a duskblade will be tripping and charging builds, though a Duskblade can play around with these tools while keeping their advantages; you get to Shock Trooper feat later than a dedicated fighter, but with the spells in your repertoire, you more than make up for it with superior mobility and additional defense.
Their big ability is Channel Spell, letting them just cast a touch spell as part of the attack actions, so they don't have to worry about casting defensively or holding it for a turn while waiting for someone to discharge it onto, letting them shank a motherfucker and electrocute him at the same time. This leads to fun shenanigans like using Vampiric Touch to attack and heal at the same time, or Abrupt Jaunt to teleport people off cliffs. They can even discharge touch attack spells against multiple targets if they know feats like Whirlwind Attack for horde rape. Add on the Arcane Strike feat to turn their many spells per day into even more damaging attacks, and duskblades can hit super hard.
Making it Work
On the surface, duskblade is actually a pretty front-loaded class that can go all the way to 20th level without much fuss. You'll actually only have a few dead levels, and they are almost worth waiting for some of the good stuff later. You start with pretty great combat ability (good BAB, d8 hit points, and all the proficiencies you'll ever want), and you even get some decent utility SLAs and skill choices.
First, let's talk about your spellcasting. It looks like it kind of sucks, but it's not that bad. You'll only get to 5th-level spells, but you can get a fucking ton of those spells to cast per day, capping out at around 10 of them each spell level at the top character levels. Your spell list seems depressingly limited, but most of them are actually really good fundamental spells you might take elsewhere for a gish: chill touch, ray of enfeeblement, resist energy, and shocking grasp are all essential stuff for melee-based casting. If you're up to digging in splat there are a few ways to add spells to your spells known (Most of the same tricks that Warmage uses) and go beyond your niche, which really improves your versatility.
After that, your class abilities are what's clutch here. You start with casting in light armor, and get better at casting in heavier armor over time. You get to cast in medium armor pretty fast, and heavy shields not long after that. The real fun, however, is Arcane Channeling, which lets you use a standard action to deliver a standard action attack spell through a weapon attack all at the same time; at 13th level, you can do this with a touch spell as a full attack and the spell will apply to every target you hit. Note that arcane channeling doesn't specify that it only works with duskblade spells, but rather with any spell you can cast. You also get Combat Casting for free really early as a bonus feat.
Needless to say, these factors alone make duskblade a gish-in-a-can for anyone who wants something that's not aiming for min-max gameplay. It is considered a Tier 3 class, though its very focused spell list mean it may be considered a tier 4 class.
Anyone who didn't fail high school math can see that while a duskblade is an admirable class by itself, it's a fucking beast for a multiclass gish build. It works better with wizard than sorcerer due to MAD (just as paladin works better with sorcerer for gish), but it's honestly not a bad choice to go sorcerer for the extra slots per day. Don't forget, though, wizards have more flexibility, allowing you to swap up your abilities day by day.
Keep in mind that you aren't a God-wizard, so you can't do all the things. You are built to be a melee wrecking machine, and every spell and feat you get should stick to that premise.
Past that, the standard gish build here is duskblade, wizard, spellsword (only one level, but it's worth it since you get BAB and casting progression), and abjurant champion (the must-have prestige class of every modern gish). Usually this looks like duskblade 2/wizard 4/spellsword 1/ab-champ 5, taken in pretty much that exact order. Abjurant Champion is great because you can throw off abjurations faster, they last longer, you can burn spell slots off for bonuses, and you can get CL equal to BAB if it helps.
Your last 8 levels are up to you: Knight Phantom from Eberron is considered one of the best (and makes you into an arcane dragoon cavalryman, which is pretty cool), but if nothing else, you can just take eldritch knight (you'll get to 9th-level magic at the end, a few levels later than an actual caster. You can go specialist (or even focused specialist, if you dare), because you're never going to be as awesome as a dedicated spellcaster, so focusing on more battlefield magic is probably a good deal.
You can swap out wizard for sorcerer for more slots and less flexibility, but that's not really that bad of a deal. As a gish, you'll probably be casting most of the same spells over and over anyway; why fight it? Avoid the stalwart sorcerer ACF; you won't be around long enough to benefit from more hit points, and the penalty to your spellcasting hurts more in the long run.
Stay away from anything that stunts casting progression as much as humanly possible. You can afford a single level of loss there, but past that, you can't justify losing spellcasting ability (since it fuels your other stuff).
If you sacrifice good BAB for average, there are some interesting choices for finishing your last 8 levels. The best of these is Sacred Exorcist, which is open to arcane casters. You may think that this seems weird - average BAB doesn't seem to do much for your battle ability - but this prestige class gives you access to turn undead ability independently of other sources. This means you now have access to DMM tricks, burning turn attempts to fuel metamagic.
Another interesting choice for a prestige class is the Jade Phoenix Mage from The Book of Weeaboo Fightan Magic, since it has full BAB and martial manuevers, although it sacrifices spellcasting progression at two levels. Good ways of doing this are warblade 1/duskblade 5/Jade Phoenix Mage 10 (to get the 12 hit points at level 1, if you don't mind giving up another level of spell progression), duskblade 5/maneuver using class 1/Jade Phoenix Mage 10 (to get second or third level maneuvers with only one level of crusader, swordsage, or warblade, again, if you don't mind giving up another level of spell progression), or duskblade 5/Jade Phoenix Mage 10 with three feats if you don't want to sacrifice another level of spellcasting progression (although a feat might be better spent on an exotic weapon proficiency that one of your disciplines requires).
If you're going straight duskblade, or building a gish with sorcerer as the casting side, take Arcane Strike at 9th level (or whenever it is you get access to 3rd-level spells). This will let you sack slots for a bonus to attacks as well as extra untyped d4 damage, based on the level of spell slot you sacked, for every attack you make that round. This includes full attacks, attacks of opportunity, anything you get from an ally's spell or abilities, etc. Oh, and it activates as a free action, though it has to be on your turn.
This works slightly better for duskblade due to the massive number of spells they can throw into this feat, but it still works amazingly good for sorcerer-based gishes. You can get bigger bonuses with higher-level spell slots that way, if your spells just can't do much to a given enemy. So instead of moping about how your fireball doesn't do shit to a red dragon, you just grin and start pumping spell slots into full attacks every round. For the best effect, use Arcane Strike with arcane channeling to hit something with vampiric touch: the temporary hit points can save you when the dragon tries to full attack you in return, and your AC isn't quite high enough to stop all of them. Or, use bestow curse or enervate to truly fuck over the enemy by giving them enough penalties to make them ineffective.
In the above example, as a 10th-level duskblade/sorcerer/spellsword/ab-champ, you have BAB +8 and 4th-level spells. If you take a move and a standard action to use arcane channeling with vampiric touch, you could make a single attack at +12, which does base weapon damage + 4d4 untyped, plus another 4d6 damage that gives temp hp equal to the damage done. You still have a swift action you could use for spells that let you turn invisible, fly, or boost movement for that round.
After that, you have a bit of a choice to make: you can get more fighter-style feats to enhance melee tricks, or more spellcasting feats for that side of things. You can't be as super-tricky in melee as a properly built fighter, but charging or tripping are certainly within reach; you can get Shock Trooper online pretty easy, or build up all the tripping feats you need. It may even be possible to make some builds that don't work with fighter work better with a duskblade; a Shield Charge build benefits because you can use arcane channeling with shield bashing, and some feats allow a shield bash to do even more bullshit (such as free tripping and other stuff).
If you go for more spellcasting, you're looking at two things: metamagic, and reserve feats. Metamagic is pretty self-explanatory; you want stuff like Extend (which will, in fact, stack with the ab-champ class feature that doubles abjurations), Silent, and/or Still. If you are a sorcerer gish, consider Versatile Spellcaster, so you can do more with your spell slots. Really, though, reserve feats can be the real deal here. The best of these is Minor Shapeshift, which requires you to have a good 4th-level polymorph subschool spell (like, uh... polymorph) available. You should already have polymorph because of how Goddamn broken it is for a gish, but Minor Shapeshift means that so long as it's still available for casting, you can give yourself a nice list of bonuses ranging from temp hp, a slight bonus on melee damage, claw attacks (which do respectable damage and are never a bad thing when your weapon gets fucked up), and bonus speed (to any movement mode, no less). Arguably, the second best choice is Summon Elemental. Yes, the elemental is pretty weak compared to anything a spell could summon... except you can summon these little elementals for free with the feat, and used them in a number of clever ways. Pop one up in front of you to break an enemy charge; use them to gain flanking bonuses; open doors and chests and stuff without worrying about traps; scout out certain areas (including the other side of a wall, if you summon up an earth elemental). Other useful reserve feats are Blade of Force, Borne Aloft, Dimensional Jaunt, Hurricane Breath, and Mystic Backlash.
There is one last line of feats that can be of enormous use to a duskblade: draconic heritage feats. You need access to the Draconic Hertiage feat itself, which you can get with the Dragontouched feat. (Alternately, if you play a race that makes you dragonblooded, and you take levels of sorcerer, you can swap having a familiar for getting Draconic Heritage.) Once you unlock the heritage feats, a whole list of neat abilities opens up. Most of them flat-out give you a bonus ability for casting a spell, ranging from swift-action claw attacks, to short-range flight, to a lesser version of frightful presence. Others let you sack spell slots for immediate-action save bonuses or standard-action breath weapons. And some give you flat bonuses active all the time, such as better perceptions, or even more spells known (which is one of the few ways a duskblade can add spells to their list). Since the former two types of feats scale up with the spell levels you have access to, they are overall considered "better" for a duskblade. Draconic Breath in particular gives a duskblade interesting options when mixed up with feats like Entangling Exhalation or Exhaled Barrier.
Keep in mind, whatever choices you make, duskblades only get limited amounts of feats for their career. Map out your choices and combos in advance, and try to be ready to cover situations where one attack style won't work, so you fall back on another. For example, if a duskblade gets chill touch, ghoul touch, and enervation, undead are going to be a problem; if they have Arcane Strike and/or Draconic Breath, they can simply choose to sack spell slots to do something more effective.