This is an adaptation of the Witch Knight subclass by LaserLlama, as close to rules as written as I could get it:

Compatibility Framework support implemented by the amazing Xarara, in Compatibility Framework Subclass Patches
Alternative compatibility support in Subclass Compatch

This subclass has progression to level 20. You don't need a mod to increase the level cap, you just won't get the level 13-20 features

Multiclassing with Warlock (or another class/subclass that uses Warlock Spell Slots) will result in only having the spell slots from whichever has the highest level slots unlocked. This is not a bug, this is a consequence of how Warlock is designed.

Subclass Features:

Sanguine Offering:
3rd-level Witch Knight feature
Once per turn, when you hit a creature with a melee weapon attack, you can expend one of your Fighter Hit Dice as part of the attack to deal an additional 1d10 Necrotic damage to the target, in addition to the normal damage of your weapon.

- Implementation Notes:
- Since Hit Dice don't exist in Baldur's Gate 3, I've instead given it charges equal to half the Fighter's level, restored on Long Rest
- Sanguine Offering: deals weapon damage +1d10 Necrotic damage, usable once per turn; does not use charges on miss
- Progression:
- Level 3: 1st Sanguine Offering Charge
- Level 4: 2nd Sanguine Offering Charge
- Level 6: 3rd Sanguine Offering Charge
- Level 8: 4th Sanguine Offering Charge
- Level 10: 5th Sanguine Offering Charge
- Level 12: 6th Sanguine Offering Charge
- Level 14: 7th Sanguine Offering Charge
- Level 16: 8th Sanguine Offering Charge
- Level 18: 9th Sanguine Offering Charge
- Level 20: 10th Sanguine Offering Charge

Pact Magic:
3rd-level Witch Knight feature
The bargain you have struck with your Patron has granted you the ability to cast spells, much like a Warlock does.
Cantrips: You learn one cantrip of your choice from the Warlock spell list. Upon reaching 10th level in this class you learn one additional Warlock cantrip of your choice.
Spell Slots: The Witch Knight Spellcasting table shows how many spell slots you have, and the level of those spell slots. All of your spell slots from this feature are the same level. To cast one of your Warlock spells of 1st-level or higher, you must expend a spell slot. You regain all expended spell slots when you finish a short or long rest.
Spells Known of 1st-Level and Higher: You learn two 1st-level spells of your choice from the Warlock spell list. The Spells Known column of the Witch Knight Spellcasting table shows when you learn more Warlock spells of 1st-level or higher. A spell you choose must be of a level no higher than what's shown in the table's Slot Level column for your level.
When you gain a level, you can choose a Warlock spell you know and replace it with another spell from the Warlock spell list, which must be of a level for which you have spell slots.
Spellcasting Ability: Charisma is your spellcasting ability for your Warlock spells. 

- Implementation Notes:
- Rules as written
- Progression:
- Level 3: Gain 1 Warlock Spell Slot, 1st level; Learn 1 cantrip from the Warlock spell list; Learn 2 spells from the Warlock spell list, 1st level
- Level 4: Gain 1 Warlock Spell Slot, 1st level
- Level 5: Learn 1 spell from the Warlock spell list, 1st level
- Level 7: Warlock Spell Slots upgraded to 2nd level; Learn 1 spell from the Warlock spell list, 2nd level
- Level 9: Learn 1 spell from the Warlock spell list, 2nd level
- Level 10: Learn 1 cantrip from the Warlock spell list
- Level 13: Warlock spell slots upgraded to 3rd level; Learn 1 spell from the Warlock spell list, 3rd level
- Level 17: Learn 1 spell from the Warlock spell list, 3rd level
- Level 19, Warlock Spell Slots upgraded to 4th level
- Spell List:
- Cantrips:
- Blade Ward
- Bone Chill
- Eldritch Blast
- Friends
- Frostbite
- Mage Hand
- Mind Sliver
- Minor Illusion
- Poison Spray
- Sword Burst, range increased slightly to 2m
- Thunderclap, range increased slightly to 2m
- Toll the Dead
- True Strike
- 1st level:
- Armour of Agathys
- Arms of Hadar
- Cause Fear
- Charm Person
- Expeditious Retreat
- Hellish Rebuke
- Hex
- Protection from Evil and Good
- Witch Bolt
- 2nd Level:
- Cloud of Daggers
- Crown of Madness
- Darkness
- Enthrall
- Hold Person
- Invisibility
- Mind Spike, effect changed slightly to conform to BG3's invisibility rules
- Mirror Image
- Misty Step
- Ray of Enfeeblement
- Shadow Blade, no thrown property due to the blade being inventory-bound
- Shatter
- 3rd Level:
- Counterspell
- Fear
- Gaseous Form
- Grant Flight
- Hunger of Hadar
- Hypnotic Pattern
- Intellect Fortress
- Remove Curse
- Vampiric Touch
- 4th Level:
- Banishment
- Blight
- Dimension Door
- Raulothim's Psychic Lance, naming target component excluded

Otherworldly Step:
7th-level Witch Knight feature
You can use Eldritch power to slip through cracks in reality. When you use Second Wind, you can teleport up to 60 feet to an unoccupied space you can see. If you appear within 5 feet of a creature, you can make one weapon attack against it.

- Implementation Notes:
- Almost rules as written
- Progression:
- Level 7:
- When you use Second Wind, Otherworldly Step will unlock as a free action in the bonus bin & stay until the end of the turn
- After teleporting, any enemy within 2m range of the landing point can be attacked as a free action, also in the bonus bin
- Range was extended slightly from rules as written due to some funky hitbox behavior

Enchanted Strikes:
10th-level Witch Knight feature
The Eldritch magic within your blood seeps into your weapon strikes. When you hit a creature with a weapon attack, that creature has disadvantage on the next saving throw it makes against a spell you cast before the end of your next turn.

- Implementation Notes:
- Rules as written
- Progression:
- Level 10: Gain the Enchanted Strikes passive, a direct copy of Eldritch Strike, just like the feature itself

Greater Offering:
15th-level Witch Knight feature
You can restore your own vitality by inflicting pain on others. When you use Sanguine Offering, you gain temporary hit points equal to the necrotic damage dealt to the creature.
Temporary hit points you gain from this feature last for 1 minute, or until you gain temporary hit points again.

- Implementation Notes:
- Almost rules as written
- You will gain 1d10 temporary hit points, not the same as the damage dealt
- Temp HP will last until long rest or expended
- Progression:
- Level 15: Sanguine Offering replaced by Greater Sanguine Offering

Profane Sacrifice:
18th-level Witch Knight feature
You can offer the death of your foes to your Eldritch Power for enhanced abilities. As a reaction when a creature dies within 30 feet of you, you can regain one of your expended Pact Magic spell slots, or 1d4 of your expended Hit Dice.
Once you use this feature you must finish a short or long rest before you can use it again.

- Implementation Notes:
- Almost rules as written
- Only works on creatures you kill
- Option to recover Spell Slots/Sanguine Offering Charges will stay in the bonus bin until short or long rest.
- Progression:
- Level 18: After your first kill per short rest, Profane Sacrifice: Restore Spell Slot & Profane Sacrifice: Restore Sanguine Offering Charges will unlock in the bonus bin, costs a reaction, you can only use one