The Occult Ritualist is a strange one, practitioners of this style of magic have had some sort of corrupting mystical or magical influence early on in life and strive to seek out and master all forms of spell casting. However, they lack the dedication to become a Wizard, Eldritch Knight or Arcane Trickster, the performance ability to become a Bard, the devotion to become a Cleric or Paladin, the appreciation for nature to become a Druid or Ranger and they possess no inherent magical nature needed to become a Sorcerer, many are actually people who were duped into selling their souls to Warlock patrons for material gain and now find themselves in a situation where their patron has discarded them and has no interest in providing them with the magical powers to be a Warlock. This lack of ability usually becomes replaced by a need for overcompensation and severe character flaws, many common folk would, at best, view them as charlatans akin to Victorian era spiritualists and modern day faith healers and at worst, view them as necromancers, demonologists or diabolical  practitioners of forbidden arts.

An example of an Occult Ritualist is the story of Sir Pewter Hooper, Bards tell stories of this bogeyman figure and how he was a human merchant who became the unwitting pawn of a fiend in his early twenties and made a bargain for mercantile power and wealth. A few years later, his business rivals grew tired of his stranglehold on the marketplace and hired assassins to remove Sir Hooper, the assassins failed and Sir Hooper was contacted by the fiend again who offered him a new deal...in exchange for his soul he would be granted Immortality! Sir Hooper agreed. The next assassin was successful and ended Sir Hooper's life, but, Sir Hooper found himself reincarnated in the Nine Hell's and the true bargain became known, his wife and children would now be held prisoner and suffer an eternity of torment as an additional cost for Sir Hooper's immortality, Sir Hooper heard his family being tortured shortly before being plane shifted to a random Prime Material Plane to continue his existence. Sir Hooper still ages and is susceptible to illness, injury and disease and general misadventure, each time he dies he reincarnates in the Nine Hell's before being plane shifted to another random Prime Material Plane where he resumes his life at the same age he was when the immortality pact was forged (roughly 40's but accounts vary). He has apparently lived through multiple lifetimes and sort out all known forms of spell caster in an attempt to end what he now knows is a curse but most have cast him aside when they find out the reason for his predicament, supposedly, even the Gods have turned their backs on him and other Patrons see little use in helping him as he has nothing of value, his soul has already been sold after all, his sole hope now is to locate a Wish spell powerful enough to undo the pact he made. He was put on the path of the Occult Ritualist by an Archefey who granted Sir Hooper a form of the Pact of Tome in exchange for performing some unknown act, since then Sir Hooper has copied and corrupted numerous spells in an attempt to become powerful enough to end his life and the pact he made, unknown to him, and a source of endless mirth for the Fiend and Archefey he has dealt with so far, the pact of tome he was offered will never grant him enough power to perform the Wish he needs. Tales tell of his many exploits, deeds and misdeeds, few accounts portray him kindly and most accounts tell of a ruthless individual with few scruples and a willingness to do whatever is need to achieve his goal.

DM note: Having this as a Fighter subclass may seem a little odd but it is written this way from the point of view of "What if?"

What if you sold your soul for material wealth? or What if your Pact was worded badly? or What if your Patron really, really didn't care for you?

Occult Ritual

At 3rd level, you have a working knowledge of magic and its various components. You gain a spell book and learn two level 1 spells with the ritual tag and one level 2 spell with the ritual tag. As you advance in level you can add further ritual spells to you book, these spells can come form any spell list but must be of a level equal to or lower than your proficiency bonus. To add a spell you need a spell scroll and scribing this into your spell book takes 2 hours and costs 100gp per level of the spell. The cost represents material components you expend as you experiment with the spell to master it, as well as the fine inks you need to record it.
For all spells gained through this class your Spell Casting Ability is Intelligence.

Ritual Magic

At 3rd level, your ritual craft now allows you to ritualise certain non-ritual spells. you learn one level 1 spell which does not have the ritual tag and can cast it as a ritual.
Spells you choose for this ability have the following restrictions/adjustments:
If the spell has a casting time of 1 Action, the casting time becomes 10 minutes.
If the spell has a casting time greater than 1 Action, it becomes double the casting time or 10 minutes which ever is greater.
You can only choose spells for this ability that have a duration greater than 1 round.
You cannot choose a cantrip.
Spells you select still require any material components listed for the spell.
Spells cast through this ability are only cast at their lowest level.
You gain one additional spell of this nature each time you gain a level in this class (to make a total of 17 spells known at 20th level). These spells are deemed to be ones that you have pieced together from observing other spell casters or by experimenting with magical rituals.
The maximum level of spell you can learn using this ability is equal to proficiency bonus but you can choose a spell from any class but it cannot be a cantrip.

All spells you learn with this ability gain the Ritual tag, should you teach them to other spell casters the spells revert back to their normal version unless that spell caster is also an Occult Ritualist.

Ritual Mastery

At 7th level, you have mastered the art of ritual magic and have learned to empower your rituals. All rituals you cast benefit from the Sorcerer Meta-magic: Extended Spell. You also learn a ritual to Animate Undead, you learn Animate Dead and can cast it as a 10 minute ritual, you can also choose to learn Create Undead and Danse Macabre through your ritual magic ability.

Anti-Magic Ritual

At 10th level, your knowledge of ritual magic gives you insight into unraveling and dissolving magical energies. You gain Dispel Magic as a ritual spell. Cast in this way it has a range of 30ft and a casting time of 10 minutes after which you need to succeed on a Spell Casting Ability Check (DC equals 10+ the level of the spell you are trying to dispel), if you succeed the spell ends.

Ritual Trap

At 15th level, you learn how to bind magical rituals and turn them into magical traps. You gain the level 3 spell Glyph of Warding and the level 7 spell: Symbol. The casting time for Glyph of Warding becomes 2 hours and the casting time for Symbol becomes 10 minutes and they still require their material components.

Ritual Specialisation

At 18th level, you have achieved such mastery over certain rituals that you can cast them at will. Choose a level 1 spell and a level 2 spell from your ritual spell book. You can cast those spells at their lowest level at their normal casting time and without needing to expend a spell slot. You cannot cast these spells at higher levels. By spending 8 hours in study, you can exchange one or both of the spells you chose for different spells of the same levels. Spells cast through this ability do not benefit from the Ritual Mastery ability,