A minor alien power found your world and decided to create a foothold. To expand its influence and attain more power it offered you a deal.
Your battle proves and enigmatic nature attracted attention of some alien force, though, you are servant of nobody. You accepted the deal this entity offered you and bound your soul to this entity, but you have your own goals and you will never be enslaved to others. You take more and more power though your contract and eventually you may rival or surpass your patron. Your soul is stronger than steel and your patron can barely hold it, and as it wrestles your patron more more and more power, you learn how to dominate others not only through strength, but also thought powers you have taken from your patron.

Pact Magic

Spellcasting
When you reach 3rd level, you augment your martial prowess with the ability to cast spells. You can learn 1 spell from warlock's spell list at level 3 and 1 additional spell every 2 levels in this class.

Cantrips
You learn one cantrips of your choice from the warlock's spell list. You learn an additional warlock's cantrip of your choice at 10th level.

Spell Slots
The Alien Knight Spellcasting table shows how many spell slots you have to cast your warlock's spells of 1st level and higher. To cast one of these spells, you must expend a slot and each spell casted is casted at its highest level possible. You gain 1 spell slot at level 3 and 1 additional spell slot at level 10.
You regain all expended spell slots when you finish a short rest.

Spellcasting Ability
Charisma is your spellcasting ability for your warlock's spells, since you learn your spells through the alien pact. You use your Charisma whenever a spell refers to your spellcasting ability. In addition, you use your Charisma modifier when setting the saving throw DC for a warlock's spell you cast and when making an attack roll with one.
Spell save DC = 8 + your proficiency bonus + your Charisma modifier
Spell attack modifier = your proficiency bonus + your Charisma modifier

Soul-bound Familiar

From level 3, gain bonus hit points equal to your fighter level. Increase your hit point maximum by 1 whenever you gain a level in this class from this point onwards.
In addition, you learn how to cast Find Familiar spell and it will not count towards maximum amount of spells you can learn, but you can only cast this spell as a ritual. A familiar summoned this way will share your hit points, so instead having individual health pools, you and your familiar will share your hit points. If you get reduced to 0 hit points, then your familiar will vanish.
You can choose the form this familiar appears as, but its creature type will be an aberration.
A familiar summoned this way will be able to attack using its own action. Whenever you declare an attack action with a weapon, then you can use your familiar's reaction to cause your weapon attack(s) to originate from your familiar instead of you.

Master's Avatar

From level 7, you learn how to cast Summon Aberration spell and it will not count towards maximum amount of spells you can learn.
The creature you summon via this spell will be a copy of the alien creature which who you made your magical pact with.
When casted using the feature, Summon Aberration will not require concentration. The summoned creature will lasts for up to 1 hour, until it gets killed, until dismiss it as an action or until you cast this spell again.

Dominion Over Domination

From level 10, you learn 1 additional cantrip and 1 additional spell from warlock's spell list.
You also become immune to Charm effects, and other creature can not longer read your mind or communicate with you telepathically, unless you allow it.
Your familiar summoned through Soul-bound Familiar feature now will have same AC as yours and its attack rolls now benefit from your proficiency bonus.

Mantle Of Domination

From level 15, your will overrides your Patron's will and you take control over them.
You learn Charm Person spell and it will not count towards maximum amount of spells you can learn. When used through this feature, then you can cast Charm Person as a bonus actions, you will require no components to cast it, and it will last for up 12 hours.

Alien Overlord

From level 18, an Aberration summoned via Master's Avatar feature will last until killed, dismissed or until you cast Summon Aberration again. It also gains bonus to its attack rolls equal to your proficiency bonus.
In addition, any damage you or your Familiar takes can be redirected onto your summoned Aberration.