This winds up getting away from the idea of the hulking thug rogue/barbarian; the solitary barbarian level is used to grant mobility and agility rather than strength and endurance. That said, this is a very vicious combat build with lots of dirty tricks, very capable of pouncing on opponents unawares and tearing into them in an unholy frenzy.
The Assassin
For a low-optimization, low-magic campaign, where you want to focus on stealth and skills but still be able to deal the hurt, you could do a lot worse than the core assassin prestige class. It gets a number of low-level spells, so it should not be a problem for low-magic, but if there’s little magic available, you definitely want what little you can get.
If alignment is an issue, consider the avenger: it replaces the Evil requirement with a Non-Chaotic requirement, which is often much more amenable to characters.
Spells
Spells are the reason you go this route. The assassin spell list is quite solid, for as small a list as it is, and it will likely fly well in a low-magic game.
This recommendation does assume that you have access, at the least, to Spell Compendium. The core assassin spell list leaves a lot to be desired. If you want to use the avenger, also make sure your DM is OK with adding the Spell Compendium assassin spells to the avenger list. That’s in line with the guidance in that book for non-core spellcasting classes on pg. 3, so it shouldn’t be a problem, but it’s a conversation you should have.
Psionics
The psionic assassin from Secrets of Sarlona also exists, and adds the very interesting option of using Expanded Knowledge for psionic minor creation, which is an excellent way to get poisons. That said, unless your DM allows other assassin spells to be converted into psionic counterparts, it’s not a great option. If you can, then you might as well?
Death Attack
Don’t focus too much on Death Attack; at best you can attempt it during a surprise round after spending three rounds studying your target out of combat. Attempting to hide and wait three rounds mid-combat is always a bad play.
Poison Use
Poison Use is meh; if you want to use poison, you want the Master of Poisons feat from Drow of the Underdark anyway. That said, Master of Poisons is a quite-solid feat, and Craft (poisonmaking) can get you poison at a huge discount (if you have a source of materials, ⅙ market price). Poisons provide a way to apply debilitating effects along with straight damage. See the Arsenic and Old Lace handbook for more details if you’re interested in going for poisons.
Again, if you are avoiding being Evil, you have to clarify something with the DM: poisons, according to Player’s Handbook and Dungeon Master’s Guide, are not evil, though usually illegal (and always dishonorable, which is why they are barred to paladins). Book of Exalted Deeds, however, says they are actually Evil – and then gives a definition/explanation of how and why they’re Evil that would include, for example, the natural poison of the couatl, that is, a notably Good creature. Book of Exalted Deeds is not a good book. Just make sure, if you don’t want to be Evil, that your DM agrees that Book of Exalted Deeds is stupid, and poisons aren’t any more inherently evil than swords.
Hide in Plain Sight
It’s a long time coming, but getting Hide in Plain Sight, along with taking the Darkstalker feat from Lords of Madness, will prevent a lot of the major ways to simply shut down your stealth. Highly recommended.
The Black Dog
This prestige class from Dragonmarked stacks with assassin levels for Death Attack DCs, and has a couple of excellent abilities aimed at poison. I’ve decided that poison is a good direction to take this, hence its appearance here.
The black dog requires the Mark of Hospitality feat, which in Eberron is limited to halflings. This is a matter of setting fluff, so if you aren’t playing in Eberron, the Mark of Hospitality might be available to half-orcs. On the other hand, I actually like halfling a great deal more than half-orc here. It does mean a shift from Strength to Dexterity which you may find undesirable, but half-orc is quite weak, and the halfling’s size is basically all to your benefit (average of −1 on damage rolls, but +1 attack, +4 to stealth, and +1 to AC). The existence of the strongheart halfling (Forgotten Realms Campaign Setting) really seals that deal.
Suggested Build
Strongheart Halfling Rogue 3/Barbarian 1/something 1/Black Dog 5/Assassin 10
At Rogue 2, the Penetrating Strike alternate class feature (Dungeonscape) is key, and at Barbarian 1, you really want the Lion Spiritual Totem alternate class feature (Complete Champion) to get Pounce instead of Fast Movement (you’re not a charger per se, but you do want to maintain mobility and full-attacks, and pounce is a really good way to do that).
I’d replace Rage with Ferocity, though I’d also consider Whirling Frenzy.
The something 1 I’m not sure about; barbarian 2 is OK but not great, rogue 4 is fairly mediocre...
Some options that should be no problem:
Fighter. You want Weapon Finesse at this point, so that’s a use for your bonus feat, and the Hit-and-Run Tactics alternate class feature from Drow of the Underdark trades some armor and shield proficiency you probably won’t use for +2 to initiative and Dex-to-damage vs. flat-footed foes, which is awesome.
Ranger. Simple: full BAB and 6+Int skills. Boring, but functional.
Anything that gives Sneak Attack +1d6 at first level. Again, boring, but functional.
Some options that probably aren’t allowed, but could be awesome:
Cleric. Best single-level dip in the game, bar none. Could be worth it even if you get none of its spells. Domain granted powers are awesome, and failing that you can trade them for Devotion feats (Complete Champion), many of which are excellent. Turn/Rebuke Undead is excellent for Divine feats, including the amazing Lolth’s Caress (Drow of the Underdark). While a halfling who worships Lolth is weird, with this build’s interest in poison it’s not a stretch to worship the goddess of spiders. Or if Lolth doesn’t exist in your setting, maybe you can just ignore that requirement.
Ardent or Psion. One level is sufficient to manifest psionic minor creation, which will get you a plant-based poison that lasts an hour. Not a bad deal at all.
Swordsage. Cloak of deception provides instant flat-footing for an enemy, once per encounter, while shadow jaunt provides great mobility. Wolf fang strike and sudden leap allow you to continue to dual-wield effectively even when you’re forced to move and cannot charge. If you later take Martial Stance, you can snag assassin’s stance for +2d6 Sneak Attack damage.
Anyway, you definitely want the Craven (Champions of Ruin), Darkstalker (Lords of Madness), Master of Poisons (Drow of the Underdark), and Weapon Finesse feats. Plus, black dog requires Mark of Hospitality (Eberron Campaign Setting).
The feats Extra Rage (Complete Warrior) and Wild Cohort could be very nice for you, if you can squeeze them in (maybe later, or if you take a Fighter level and have another feat to play with). Note that I don’t recommend Wild Cohort so you have a pet fighting alongside you: I recommend it so you have a reliable, cheap poison dispenser following you around. Unfortunately, both feats are kind of lackluster by the time they fit into the build I am proposing.
Anyway, your feats may look like this:
- Bonus racial feat: Weapon Finesse
- 1st-level feat: Master of Poisons
- 3rd-level feat: Mark of Hospitality
- 6th-level feat: Darkstalker
- 9th-level feat: Craven
- 12th-level feat: Extra Rage?
- 15th-level feat: Wild Cohort?
With this combination, your attacks hit hard, and you are hard to find. You have poisons that you can use selectively, and they are very dangerous. You have a smattering of spells that will make a huge difference in a low-magic world. Seems like a pretty solid approach to me.
Best Answer
There are issues, yes. That doesn't mean it shouldn't be done, but there are things to watch out for.
In particular, it walks around some of the ways that 5e uses to balance itself. In the 5e core classes, you have noncasters, full casters, half casters, and warlocks as something of a special exception. The core of spellcasting is shared pretty directly between the various casting classes, to the point that caster levels can stack for spell slots and it works. By contrast, 3rd edition and 3/5 went kind of crazy with embracing of wildly varied magic systems, and the binder system was out towards the wacky side.
Vestige effects are different than other things. They were intended to be. That means that if you don't know the 5e design assumptions cold, you could break one or more of them very easily, without thinking you're doing anything all that significant. Just for an example off the top of my head, "bounded accuracy" and "always-on stackable debuff auras" aren't necessarily going to mix well.
Vestiges stack with each other in strange and not necessarily easily predictable ways. If you want to give your players a reasonably large library of vestiges to choose from, with interesting and flavorful effects, then it's going to be tricky to make sure that you aren't unintentionally handing them more power than you intend. Further, it can be surprisingly easy to produce a vestige that suddenly becomes class-defining. Low-level summons in particular are much more useful in 5e than they were in 3.x, and Binder had a fair chunk of summoning access.
Or, to put it another way, you can do this, and it might well be worth it, but both you and the player ought to be aware going in that this is a class that's going to require some additional help to keep balanced, and will probably require tweaks to keep from being too powerful or too weak, even more than is the case with most homebrew. Hey, vestiges are fickle, right? Play it right, and it could even add to the flavor of the class. One thing that might help with that - if you can whip up something reasonably decent for the class features, you should be able to manage the balancing purely through tweaking various vestiges.
Also, the structural design assumptions of 5e may not jump out at you immediately, but they're very different than those of 3.x, and they're pretty important. I don't personally know them well enough to teach them all to you, but if you don't know about things like Bounded Accuracy, you should try to learn them before you move on to trying to a homebrew on the complexity level of the binder.