Baldur's Gate III Windows, Mac game

"Through ancient rituals and sacrifice, the Cultist earns the attention of damned spirits and Outsiders, and is drawn inevitably into the service of the dark gods"

The dark cultist has existed throughout the various editions of D&D as a plot device. If they are given a class in supplements that use them, it will generally be an evil Cleric or maybe a Warlock. This works for some kinds of cultist, and you could use an evil Druid as another possible route. But none of these really hits the spot (IMO) when it comes to the truly forbidding Witch archetype, or the Lovecraftian cultists of the Great Old Ones. These individuals aren't heroes, and manifest as something a lot more rough and unrefined.

In terms of existing DnD archetypes, the Cultist is (in terms of how they live) most comparable to a Druid, however instead of being involved with nature and its gods, they are involved with realms alien to Faerun, and their unknown, forgotten gods; in either case, they tend to live away from civilized society to properly develop their powers. The Cultist's ability to exploit dark forbidden magic makes them feel similar to the Warlock, however instead of Pacts, they learn what Larian calls Shadow Magic, and will have access to limited spell lists from this realm to choose from.

The Cultist is a roleplaying class, as their alignment would be chaotic and non-good; if not evil in their beginnings, then eventually becoming evil. They are not social creatures in any civilized way, definitely not the 'face' of a party, and would likely prefer working with their own conjured minions, or new Initiates who are still attached to society in some way. Manipulating a more socially-capable hireling to deal with others is likely the only way they could parade around Faerun without being shunned - or worse!

Abilities
The primary attribute for the Cultist is Wisdom, which is also used for their spellcasting ability. Their secondary attribute is Dexterity, for similar reasons that Wizards use it. The initial suggested default schema is:

Strength: 10, Dexterity: 15, Constitution: 13, Intelligence: 12, Wisdom: 17, Charisma: 8

Class Features
The primary class features of the Cultist are disciplines which concern their spell casting abilities:

Wisdom Casting: You use your Wisdom as the primary ability for casting spells.
Domain Spells: You gain spells from your domain. They are always prepared.
Dark Gnosis: You gain Shadow Spell Slots, which are restored on a long rest.

However, true dark cultists are created by unique life experiences, and what they learn is not presented in the same way as a normal profession, so certain abilities will only be obtained when starting the Cultist as a new character, and the below won't appear when multiclassing into Cultist:

Malefic Birth: This character's traumatic early life has made them resistant to Psychic damage.
Hexed Flesh: Warding glyphs were etched into your skin. While not wearing armor, you add your Wisdom modifier to your Armor Class.
Proficiencies: Daggers, Short Swords, Scimitars, Spears, Quarterstaves, Light Armor, Medium Armor, Shields.

Skills
Cultists may invest in Arcana, Deception, History, Insight, Medicine, Perception, Persuasion, Religion, SleightOfHand, Survival, and Stealth as their Skills with Proficiency.

Progression
Although the Cultist is limited in the maximum spell level they can cast (IV), their available spell slots advance a bit more aggressively than a Wizard. Available Cantrips and spells are restricted to those in the lists below, however additional unique Domain spells are learned at appropriate levels. Divinity Charges are gained from lvl 2 onward, and the Cultist will acquire a total of five of them by level 11.

Level 1
Dark Gnosis: two lvl 1 slots
Spells: Choose 3 Cantrips and 3 Level 1 spells.

Level 2
Divinity Charges: +1
Dark Gnosis: one lvl 1 slot
Spells: Choose one additional spell, and you may trade an existing spell for a different one.

Level 3
Divinity Charges: +1
Dark Gnosis: one lvl 1 slot, two lvl 2 slots
Spells: Choose one additional spell, and you may trade an existing spell for a different one.
Divinity Actions: Control Undead, Stark Horror
Passive: Darkvision
Domain Spells: Darkness, Silence

Level 4
Dark Gnosis: two lvl 2 slots
New Feat: Standard feats list

Level 5:
Cantrip: (Doom Bolt gains one extra bolt per cast)
Dark Gnosis: two lvl 3 slots
Divinity Actions: Conjure Shadow Wraith
Passive: Sacrificial Offering (When you kill an enemy, nearby allies gain temporary hit points equal to your Wisdom modifier).
Domain Spell: Speak with Dead

Level 6
Divinity Charges: +1
Dark Gnosis: two lvl 3 slots
Cantrip: Choose an additional Cantrip
Spells: Choose one additional spell, and you may trade an existing spell for a different one.
Divinity Actions: Shield of Darkness, Psionic Visage

Level 7
Dark Gnosis: one lvl 4 slot
Spells: Choose one additional spell, and you may trade an existing spell for a different one.
Domain Spells: Hideous Mantle, Terrible Black Void

Level 8
Dark Gnosis: one lvl 4 slot
Spells: Choose one additional spell, and you may trade an existing spell for a different one.
Passive: Superior Darkvision
New Feat: Standard feats list

Level 9
Divinity Charges: +1
Spells: Choose one additional spell, and you may trade an existing spell for a different one.
Divinity Actions: Gaze of Doom, Dark Lance

Level 10
Cantrip: Choose an additional Cantrip (Doom Bolt gains one extra bolt per cast)
Spells: You may trade an existing spell for a different one.
Passive: Blackest Wisdom: you are anointed as a sentinel of Wisdom, and have Advantage on all Wisdom saving throws. Wisdom is increases by 1, to a maximum of 20.

Level 11
Chanel Divinity Charges: +1
Spells: You may trade an existing spell for a different one.
Divinity Actions: Black Hole
Domain Spells: Dethrone

Level 12
Spells: Choose one additional spell, and you may trade an existing spell for a different one.
New Feat: Standard feats list

CantripsDoom Bolt, Guidance, Blade Ward, Resistance, Minor Illusion, Produce Flame, Mage Hand

Available Spells:
Level 1: Bane, Charm Person, Command, Cure Wounds, Curse of Regret, Dark Miasma, Dissonant Whispers, False Life, Find Familiar, Fog Cloud, Inflict Wounds, Jump, Ray of Sickness, Sleep, Wrathful Smite.
Level 2: Blindness, Calm Emotions, Cloud of Daggers, Crown of Madness, Enthrall, Hold Person, Invisibility, Mirror Image, Phantasmal Force, Prayer of Healing, Ray of Enfeeblement, Shatter, Warding Bond.
Level 3: Animate Dead, Bestow Curse, Gaseous Form, Fear, Hunger of Hadar, Hypnotic Pattern, Mass Healing Word, Remove Curse, Slow.
Level 4: Blight, Confusion, Dimension Door, Evard's Black Tentacles, Greater Invisibility, Guardian of Faith (necrotic damage), Ice Storm, Phantasmal Killer.

Starting EquipmentSorcerer's robe, Gith shortsword, other items are the usual standard stuff.

Requirements
5e Spells, ImprovedUI, Vlad's Grimoire - Spell VFX Library

Installation
Add with BG3 Mod Manager
*Ensure that Vlad's Grimoire and 5e spells and above this mod in the load order.