What's the closest RAW build possible to the Gray Mouser in 5e?

Question

I am of course referring to the legendary figure of Fritz Lieber's Lankhmar series (one of the things Gary Gygax said was his inspiration to invent D&D).

I am assuming this character is so off-the-hook awesome that I cannot see any way to make him using strictly RAW, but what's the closest we can get?

Some things to keep in mind:

• His acrobatics, stealth and persuasion are incredible (especially acrobatics)
• His swordsmanship is fantastic, in a finesse-sort-of-way (rapier?), and when combined with a successful acrobatic move he can hit almost anything
• He is devastating with a sling, e.g. can knock someone out or do great damage (a thing which seems outside the RAW in 5e)
• He has a custom-made bodysuit that enhances stealth, and he is very hard to hit in melee (almost un-hittable!) despite that he has no armor or shield beyond this
• He has some low-level arcane spellcasting abilities
• While he is not immune to being charmed, he is fairly resistant to it

Those are the essentials, and here are nice-to-haves:

• Race is human
• Magic is limited mainly to illusions (less magic than an Arcane Trickster has)
• Acrobatic-based swordplay (to-hit improves somehow when using acrobatics)
• Can use a sling effectively at longer range, compared to a typical character

There may be other features I am overlooking from Lieber's material, but those are the high points. What's the closest you could come in RAW?

Answer 1

A human Fighter/Rogue (Arcane Trickster) of at least 8th level total, perhaps level 14 or higher

According to the Q&A with Gary Gygax, part 5 forum thread in 2004:

The Thief was based on Jack of Shadows (Zelazny) and Cugel (Vance) with a touch of REH’s Conan, rather than solely on the Gray Mouser. Mouser was too good a swordsman to serve as the pure model.

However, Gygax was talking about the D&D of his era, and the rules have changed much since then. Based on your requirements, the Rogue class (which in Gygax's D&D was called the Thief) with the Arcane Trickster subclass is a suitable match:

• Acrobatics, stealth and persuasion: All three of these are literally class skills for the rogue. The rogue's Expertise ability at 1st and 6th level allows their proficiency bonus to be doubled for these skills, and Reliable Talent (11th level) also extends this ability.
• Fantastic swordsmanship: The rogue is proficient with the rapier, as well as shortswords and longswords. D&D 5e no longer gives the fighter a higher base attack bonus than the rogue, so this class imposes no penalty.
  • Optional acrobatic swordplay: Tricky to model in D&D, but the most readily available solution is to drop at least a single level in fighter and adopt the Dueling fighting style which grants +2 to damage while fighting in his unique style with no shield. A level 3 fighter (Battle Master) gains unique and clever maneuvers, but this may be pushing it. I'm not aware of any official feat or option in this particular edition of D&D that specifically gives a combat bonus for using the Acrobatics skill.
• Devastating with a sling: The rogue has sling proficiency (as a simple weapon) and can gain Sneak Attack if he has advantage on the attack roll, even if he is not hidden from the opponent. A high level rogue can deal substantial damage with sneak attack.
  • The optional requirement of long range with a sling can be met with the Sharpshooter feat, which extends range and the ability to hit through cover. Currently, there's no rule which allows a nonlethal knockout with a ranged weapon (although any character may do this with a melee weapon).
• Custom bodysuit and unhittable: He would require magic items for this in D&D: magical leather armor, a cloak of displacement, and a naturally very high or magically-enhanced Dexterity. For a high-level character, these magic items are plausible.
  • Uncanny Dodge (5th level rogue ability) halves damage from attackers the rogue can see.
  • To meet the optional requirement of the character being human, with the human starting ability score bonus and ability score improvement at every 4th rogue level, he could still have superhuman Dexterity. (If you're willing to forgo the human requirement, an elf would also have naturally increased Dexterity.)
• Low-level spellcasting: The Arcane Trickster roguish archetype gains 1st level arcane spells at level 3, 2nd level spells at level 7, and 3rd level spells at level 13. This is low level magic for the character level, if we assume the character is below 18th level.
  • Mainly illusions: An arcane trickster who only chooses illusions, even if the class doesn't enforce this, is still a valid build. The first level spell silent image creates illusion with no sound; second level minor image alllows image or sound; the third level major image adds other sensory effects and the illusion can move around. If major image is beyond his ability then he should not be above 12th level; otherwise, level 14 or 15 would be possible.
• Charm resistance: A high-level character with high Wisdom would have high saves, and magic items may enhance that further. If it's not required to make the character a human, an elf of any sort or half-elf is resistant but not immune to being charmed.
  • To meet the human optional requirement while still resisting charms, he could spend his first feaat (variant human feat at 1st level ability) on Resilient to increase his Wisdom by 1 (which may already be high to begin with) and grant proficiency in Wisdom saves. If Gray Mouser is really high level, the 15th level rogue ability Slippery Mind gives proficiency in saving throws, but he could of course just take the Resilient feat at an earlier level.

On a historical note, according to Dragon Magazine #70 (February 1983), Gray Mouser would be a a very high level multiclass fighter/thief. This is consistent with Gygax's opinion that Gray Mouser is like a Thief but better at swordfighting.

_{Thanks to @DavidCoffron, @KorvinStormast, @DarthPseudonym and @sharur for suggestions in the comments.}

Answer 2

Perhaps a multiclass is the answer. Rogue/Bard might be appropriate.

Here's what I would do to try and create a character like this.

First off: play a Variant Human, and start with the Sharpshooter feat. This will give you that extra range and damage with the sling.

Begin as a Rogue, eventually choosing the Swashbuckler subclass from Xanathar's Guide to Everything. Take Expertise in Acrobatics and Persuasion at level 1, and in Stealth and another ability of your choice at level 6. This will give you excellent acrobatics, stealth, and persuasion. At level 4, take the Resilient feat, choosing to give yourself Proficiency in Wisdom saves, increasing your resistance to most Charm effects. Your sling and swordsmanship abilities will be represented by the ever-increasing damage of your Sneak Attack. Melee Sneak Attacks are made more easily thanks to the Swashbuckler level 3 ability Rakish Audacity, which lets you Sneak Attack if your target is the only creature (ally or enemy) within 5 feet of you. A Rapier will probably be your melee weapon of choice.

There is some room for flexibility, but you will probably want to end up with at least 13 levels in Rogue to gain the Swashbuckler ability Elegant Maneuver. This allows you to use a Bonus Action on your turn to gain advantage on the next Acrobatics check you make during the same turn. This will raise your already Expertised Acrobatics ability even higher.

To improve your swordsmanship and grant some Arcane spellcasting, I would also take several levels of Bard. By taking 6 levels of Bard and choosing the College of Swords (also from Xanathar's Guide to Everything), you can gain several benefits for this build. At level 3, you gain the Dueling fighting style, to further enhance your swordsmanship. You also gain the Blade Flourish ability, which allows you to use Bardic Inspiration to improve your attacks and can be used to represent your superior swordsmanship when combined with acrobatic skill. At level 6, you gain the Extra Attack feature, which will improve your melee capabilities even further. You also gain the ability Countercharm which allows you to use our Action to gain advantage on saving throws against Charm or Fear until the end of your next turn. This (on top of the Resilient feat mentioned earlier) fits in with being "fairly resistant" to Charm effects. From 6 Bard levels, you will also gain access to Illusion spells up to third level. You have the option to choose other spells as well, but if you want to focus on Illusions, that's your prerogative.

Your bodysuit could take the form of the Rare magic item Glamoured studded leather. This is +1 studded leather armor that can change its appearance when you use a command word. If that's not stealthy enough for you, it could be paired with the uncommon Cloak of Elvenkind which gives advantage on Stealth checks. A rare Cloak of Displacement, which gives disadvantage to incoming attacks might also be a viable option. All of these are in the DMG.

In the end, you're probably going to end up with a level 13 Rogue/level 7 Bard or level 14 Rogue/level 6 Bard. I would recommend the latter as it gives the useful Rogue ability Blindsense. The levels can be taken in any order that makes the most sense to you.

I believe this build covers most of your main and optional criteria.

Answer 3

Kensei Monk, with some other investment for magic

As a Kensei Monk, you gain access to several things that address your concerns.

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Notably, the base class Monk gets:

• Wisdom and Dexterity to AC. As long as his "bodysuit" isn't considered real armor, he will be extremely difficult to hit, despite being nearly naked.
• They naturally have an aptitude for Athletics, Stealth, and Acrobatics, and Persuasiveness can be almost obtained with an optimized Insight skill. Persuasion can be obtained via a background.
• They can spend a Ki point to grant them the Dodge, Disengage, or Dash action at the cost of a Ki point while doubling his jump distance (referencing his acrobatic prowess).
• They have enhanced mobility, and can even run up walls and across water at later levels.
• They eventually get proficiency in all saving throws at level 14, and the ability to remove any Charm or Fear effect on them by spending an action at level 7. A naturally high Wisdom score (optimized with a monk) will also increase your chance to resist these effects in the first place.

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As for the Kensei subclass specifically:

• They can choose to select specific weapons to specialize in, which can include a Rapier and a Sling.
• These specialized weapons gain increased damage and powers, making even a Sling a dangerous weapon, especially considering it's ammunition is simply a rock.
• They are naturally proficient in both melee and ranged combat, and can enhance both with powerful effects. A simple sling can deal 2d4 + 8 (13) magical damage per hit and add your Martial Arts die (1d4-1d10) once per turn. It would have + 8 to hit, and you can reroll a missed attack once per turn.

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For magic

There're two options, depending on how far you want to go.

For combat support, 3 levels into Arcane Trickster would get you some necessary illusion spells that would improve your interactions when out of combat, while also giving you a 2d6 sneak attack die each turn.

Alternatively, a Magic Initiate (Wizard or Bard) feat will require less investment and grant you things like Minor Illusion, Prestidigitation, Fog Cloud or Disguise Self. Your options would be very limited, though, and wouldn't grow beyond this point.

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For Range

As you're looking to be a human, being a Variant Human, investing in Sharpshooter, will provide the range enhancement you're looking for, and will also work well with other options, such as darts or a longbow.