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An eldritch fighter class, similar to my Faerie Irregular

Eldritch Gift
At 3rd level, you are possessed by a foreign being, or such a being already within you gives you control over a portion of its power, granting you an eldritch invocation of your choice. When you reach 7th, 12th and 18th levels, you gain additional invocations of your choice. When you gain a level in this class, you can choose one of the invocations you know and replace it with another invocation that you could learn at that level.

This does not come from a patron, and does not grant any boon. The level prerequisite for any Invocation is doubled, but uses your fighter level instead of your warlock level. Invocations that allow you to cast a spell using a warlock spell slot can be cast once per day without using a spell slot, and Charisma is your spellcasting stat for all spells you gain from eldritch invocations.

Spirit Knowledge
You gain proficiency in your choice of Arcana, History, Nature, or Religion, and learn 2 warlock cantrips of your choice. Charisma is your spellcasting stat for these cantrips.
Warlock Invocations; the main feature of the class. I feel there should an in combat bonus at 3rd level, but I guess the cantrips can count (they appear to count for the eldritch knight?).

Striking Specter
Starting at 7th level, you get a special reaction that you can take once per round turn. When you would be able to make an attack of opportunity, you can instead use this special reaction to have your possessor lash out at the creature, dealing 1d12 + your Charisma modifier damage on a hit. If that creature damaged you since the start of your last turn, your possessor has advantage on the attack.
Based on the wording of the cavalier. With a large die, considering how often you get in attacks of opportunity.

Incorporeal Possessor
Starting at 10th level, your possessor can shield you from an attack, reducing the damage to zero and healing you for 1d6 hit points. You cannot use this feature while unconscious.

Once you use this feature, you can't use it again until you finish a long rest.
Massive survival, against only one hit. Very swingy, but it grants the sort of bonus full spellcasters save their high level spells for. Far stronger on a 15 minute adventuring day, and less powerful the more the rest of the fighter's toolkit gets to kick in.

Seeing Specter
Starting from 15th level, your possessor can reach out and sense around you, warning you of things you may have missed. You have advantage on Insight (Wisdom) and investigation (Intelligence) checks.

Infused Foresight
Starting at 18th level, you can cast Foresight requiring no material components, targeting yourself, as an action. The effect lasts for 1 hour. You can use this ability once per long rest.
Constant and shorter effects. I like how they fit the theme, although I'm not sure how powerful they are.