How do you optimise a low-level vault-buster heist character?

Question

I'm going to run a campaign that starts with characters on level one and finishes on reaching level six in 998 YK. I'm looking for a complete path for the character build through level one to six. One of my players wants to play an expert at busting open magically secured and conventionally re-enforced vaults in nobility-estates from various nobles who have access to a caster with up to third level spells - either a cleric, wizard or sorcerer. At one point or another, the character will encounter each variant of the spellcasters. This PCs role is as part of a team:

Somebody else deals with getting in and out
Somebody else deals with information and resources gathering
Somebody else deals with organising combat should it occur

The group wants to avoid fighting most of the time.

The group is most likely to solve encounters via roleplaying in a conniving way. If the character additionally provides magical disguises, then that would be desirable, but it isn't necessary.

What is that optimal build that can deal with magical defences that may occur during various heists on these estates?

Consider the following restrictions:

• Only official material, Unearthed Arcana is not legal for this character creation. The choice is limited to core rules, ERftLW and one other source.

• All PHB and ERftLW races are legal. The choice of Subraces depends on the selection of sources.

• Starting attributes are achieved by the point-buy or standard array.

• Consistency is imperative: a build that only functions under unreliable circumstances or relies on later levels to function is undesirable.

Answer 1

TL;DR: you want to be a human (variant) Divine Soul sorcerer.

To start with, let's figure out what magical defenses we are dealing with.

The level 1-3 wizard spells we are up against are as follows:

• alarm
• illusory script
• snare
• arcane lock
• Nystul's magic aura
• glyph of warding
• Leomund's tiny hut
• magic circle
• nondetection

The sorcerers can't contribute anything unique to the defense, and the cleric is merely able to create different glyphs of warding which we can deal with anyway.

Dispel Magic

You are going to need access to detect magic and dispel magic at 5th level. This is your bread and butter for removing many of the defenses including the following:

• alarm
• glyph of warding
• Leomund's tiny hut

...as such, we are going to be starting as either a bard, cleric, druid, sorcerer, or wizard.

Investigation

One of the more vital skills the character will need is Investigation. Firstly this will help find mundane traps, but also it'll help with certain magical ones. Because Nystul's magical aura can disguise some of them, detect magic may not be sufficient for all defenses. This will be used to locate and avoid/dispel the following defenses:

• snare
• glyph of warding

The best way to have high Investigation is to use the enhance ability spell and have Expertise in Investigation. This can be achieved by bards, clerics, druids, and sorcerers. Bards gain Expertise as a class feature; the other classes will have to pick up the Prodigy feat from Xanathar's Guide to Everything.

Locks

Mundane locks and vaults are likely to be bypassed using thieves' tools, but since we are buckling in for dispel magic, we won't be able to get Expertise in thieves' tools (which requires being a rogue). Instead, we could use the knock spell to bypass tough locks that simple proficiency fails to surpass. The main downside of the knock spell, however, is that it makes a loud sound. As such, the silence spell is also vital to this technique.

Knock also bypasses the following magical defense:

• arcane lock

The only classes that get both the silence and knock spell are bards and Divine Soul sorcerers (learning silence through Divine Magic giving access to the cleric spell list). Divine Soul sorcerers are the better of these options since they also get access to guidance earlier than a College of Lore bard to make our basic thieves' tools proficiency that much more potent. Thankfully, Divine Soul sorcerer subclass is also in Xanathar's Guide to Everything so we can choose it and still gain the Prodigy feat for Investigation.

The remaining defenses

Because we must be a Prodigy to gain Investigation Expertise, our character race is limited to human, half-elf, and half-orc. This means that magic circle can't keep us out, so the only remaining defense that we need to get past is illusory script (in cases where the item we are trying to steal is disguised by this spell).

The only way to defeat an illusory script (which can't be dispelled without ruining the original text) is to have truesight, but the true seeing spell isn't available until level 11. The only way to get access to truesight earlier is through employing a monster that has truesight (none of the possible summons are low enough CR to be gained earlier than the true seeing spell) or finding a gem of seeing (a rare magic item).

Since truesight is out of the question without that magic item, you are better off just waiting the 10 days just in case there is illusory script when stealing sensitive documents.

The Build

Putting all of this together, we get the following build:

We start as a level 1 human (variant) sorcerer of the Divine Soul Sorcerous Origin with the Prodigy feat and the following ability scores:

• 16 in Intelligence (for that high Investigation score)
• 14 in Dexterity (for using thieves' tools)
• 14 in Charisma (since this is your spellcasting ability score)
• 12 in Wisdom (since Perception is marginally important)
• 10 in Constitution (you don't want a negative modifier in Constitution with only a d6 hit die)
• 8 in Strength (this is your least important ability; you can always use enhance ability if you need to carry something heavy)

Our background can be anything that provides you with skills you think might be important (Stealth might be a safe bet just in case even though someone else is dealing with getting in and out). We get our essential proficiencies from the following sources:

• Perception from Skilled (Variant human trait)
• Investigation from Prodigy
• Thieves' Tools from Prodigy

At level 2, we get Flexible Casting, and at level 3 we get Metamagic. Metamagic is not super important (except perhaps the occasional Subtle Spell) for your heists, so we will typically use Flexible Casting to convert sorcery points into whatever spell slots will be required for the heist (based on whatever intel is gathered by the person in charge of that).

At level 4, it is best to increase Intelligence since Investigation is so important, but you can get away with increasing Charisma to improve your spellcasting if you want. And finally at level 5, we get the dispel magic we chose sorcerer for.

The following spells are important to the heists:

• guidance (from the cleric spell list)
• detect magic (to find magical defenses that aren't obstructed by Nystul's magic aura)
• enhance ability (for Fox's Cunning usually, but also useful for Bull's Strength with heavy loot)
• knock (to open locks that we can't get through with thieves' tools)
• silence (from the cleric spell list to keep knock from alerting anyone)
• dispel magic (to bypass the more difficult defenses)

There are some additional spells that could be beneficial:

• absorb elements (in case a glyph of warding with Explosive Runes is not detected; is found in XGtE so it fits in the +1 source)
• lesser restoration (from the cleric spell list, in case a glyph of warding with a Spell Glyph is not detected; can remove certain conditions)
• find traps (from the cleric spell list, although nondetection might make this useless)

And here's a spell we don't get that would be very useful (maybe you could ask your GM if your sorcerer could learn it anyway since it is a domain spell for some clerics):

• identify (would help identify exactly what spells are affecting an object instead of just giving an aura like with detect magic)

Level 6

Level 6 is kind of up in the air. You could do a number of options, each with their own benefits:

• continue in sorcerer to get more sorcery points and spell slots to deal with better defended marks,
• Multiclass as a rogue to get Expertise in thieves' tools
• Multiclass as a wizard to gain a number of useful rituals like detect magic, identify, find familiar, and unseen servant¹

^{1: Credit to smbailey for mentioning the breadth of rituals this offers}

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Addendum: Magical Disguises

Since magical disguises are a nonessential benefit, I thought I'd mention that this build can provide these too. It can learn disguise self and alter self as a sorcerer, and glyph of warding appears on the cleric spell list.

If we create a Spell Glyph of disguise self or alter self, any of our compatriots can wear a magical disguise.

Answer 2

While I agree with the analysis of David Coffron about what is necessary for the build, there is a better way to achieve that: be a (most likely excommunicated) heir of House Kundarak.

The build

• Race: Dwarf, Subrace: Mark of Warding
• Background: House Agent(Kundarak)
• Class: Bard 6 or Bard 5 / Rogue 1
• Ability Scores: STR 8 / DEX 14 / CON 14 / INT 14 / WIS 12 / CHA 14
• lv4 ASI: Dungeon Delver

Relevant Features

Spells

First, as a Bard, this character has access to all spells that are useful to counteract the protections of a vault and are available up to level 6. These are: detect magic, identify, knock, enhance ability, silence, dispel magic

Bards can also cast detect magic, identify and silence as rituals, helping you out when low on spell slots, but not on time.

It also has access to the marginally useful spells unseen servant(ritual), see invisibility, nondetection, clairvoyance and to disguise self. Thanks to your Mark, if you want to be super sneaky you can replace an arcane lock you dispelled with your own or use it to ensure you get enough alone time with the vault (for no gp cost, 1/day).

Feat

I recommend taking the Dungeon Delver feat instead of an Ability Score Increase. A +1 to one relevant skill check does not measure up to the multiple benefits provided by the feat:

• Advantage on noticing secret doors, eg. a hidden vault door or entranceway to a vault
• Advantage on saves and resistance to damage form traps, and you will encounter a lot of them in this line of work.

While these are situational, they are quite relevant to the situations the character will get themselves into.

Proficiencies

As also identified by the other answer, the most relevant proficiencies are Investigation and Thieves' Tools. Our dwarf gets both from the House Agent Background and due to the Mark gets to add d4 to all ability checks made with both. At 3rd level Investigation gets Expertise from the bard class feature. Thus by level 6 we will have:

• Investigation +8+d4 (Advantage to find secret doors)
• Thieves' tools +5+d4

Also of note is that being a dwarf, the character gets proficiency in mason's tools and the Stonecunning feature, which might provide useful insights about the vault or the building around it.

Being a bard, you also get proficiency in Dexterity saving throws and most damaging traps ask for that.

Other stuff

We have not needed to use the +1 sourcebook allowed, and the choice of subclass has little bearing on the relevant abilities. Thus you can flavor the character with any subclass from TCE or XGtE and by level 6 can know 3 spells above what will be strictly necessary for your profession. If you choose College of Lore, at level 6 you also get +2 spells from anywhere (ie. any class or sourcebook).

Also, we get Persuasion and tinkerer's tools from the Background and any 3 other skills from our class. One of these will get Expertise at level 3 (I recommend Perception or Stealth). If you find that is not enough, go Lore for +3 skills of your choice. All other skills benefit from Jack of all Trades after level 2.

As in David's build, the 6th level is pretty much up in the air. If you go bard, you get a subclass feature (with Lore, getting the Magical Secrets is quite sweet), otherwise multiclassing to rogue is the most useful.

Answer 3

Satyr Rogue/Warlock

A good option would be a Satyr Rogue/Warlock, using MOoT as your +1 book.

Satyr gives Magic Resistance, which helps a lot against magic traps.
Rogue gives expertise with Stealth and Thieves Tools, perfect for sneaking in and for picking vault locks.
Standard Array scores: 16 Dex, 16 Charisma, 13 Wisdom, 12 Con, 10 Intelligence, and 8 Strength.

At second level, multiclass Warlock, Archfey patron. This gives you spells and a powerful charm ability. The charming is useful for if you get caught, and other means of talking and deception fail.
For spells, I would take Minor Illusion, Mage Hand, Sleep, and Charm Person, all good spells for infiltration and theft.

At third level, go Warlock again. Take these two invocations: Mask of Many Faces and Devil's Sight. Mask of Many Faces gives at will disguises. Devil's Sight gives Darkvision even in magical darkness, which allows you to foil even more magical traps.

Fourth level, take Warlock again and take Pact of the Chain. Take an Imp familiar, a spy to scout ahead with, and the Imp may change shape to be less noticeable.

Fifth level, take Warlock again and take either the Dungeon Delver feat or a +2 to Dexterity.

At level 6, take another level in Warlock. This gives you another invocation, which should be Eldritch Sight, to spot magic traps, and 3rd level spell slots. You should take dispel magic, as it can dispel any spell your enemy spellcaster can cast.

This build provides you with good sneaking, trap finding and resisting, and disguising skills, all you need for a Vault Buster.

Why this is good

This build is good because it enables you to circumvent all of the defences you might come across.

1. Locks. Locks should be no problem against your +7 (or more at higher levels) Thieves Tools modifier.
2. Spells. The main spells you will be fighting: Glyph of Warding, Alarm, Snare. You have advantage on all saving throws against magic effects, including spells, due to being a Satyr. As well, Eldritch Sight gives you at will detect magic, which you should cast at every new area, in order to know precisely where all magical traps are. This allows you to walk around, never triggering one. At 6th level, you get Dispel Magic, to get rid of unavoidable traps.
3. Failsafes. This build has many failsafes built in, in case all else fails. By sending your familiar ahead to scout, you can test for mundane traps, and look for guards to avoid. In addition, if you are caught, you can use Fey Presence to charm the guards and convince them you are supposed to be there.