Malcanthet

Few mortals even know about the existence of Malcanthet, the Queen of the Succubi, let alone embrace her sparse cults and dogmatic principles enough to gain the attention of her. Even fewer than that are so enraptured that they become her willing thrall. Thralls of such powerful fiends are the mortals who decide to dedicate their lives to the worship of their master, or in the case of thralls of Malcanthet, their mistress. In the end, a thrall's soul is not their own for it belongs only to the Profane Lady. The Succubi Queen demands absolute loyalty and expects her thralls to be as obedient as they are alluring, as vile as they are cunning, and as treacherous as they are powerful.

Thralls of Malcanthet have an otherwordly beauty to them that is both seductive and menacing. They are more prone to use cunning than entertain direct combat, seducing or charming their opponents into a false sense of friendship before betraying them outright and reveling in their foe's misery. This does not mean that thralls are without allies, however. Companionships are often viewed as necessary and are thoroughly enjoyed; few, however, are actually cherished.

This class can only be taken if you have at least 3 levels in a class other than warlock and can wear medium and heavy armor.

This subclass is based on a DND 3rd edition homebrew. (I'm not allowed by dndbeyond to link it here, but you'll easily find it on Google)

Crimson Armor

Starting at 1st level, Malcanthet grants you the ability to change the look of your medium or light armor.
You can conduct a 1-hour ritual to attune with non-magical armor. During the ritual, you can change the armor's appearance, and decide what it looks like, including color and style. For the purpose of encumbrance, the original weight is considered halved. All other stats remain unaffected.

Malcanthet may punish you by choosing herself what outfit you will get.

You may change the appearance of an already attuned item again but must conduct the 1 hour ritual for each item. The item retains its desired glamour style for as long as you are wearing it. If it is removed, it turns back into its normal form. The item resumes the last glamor it had if you put it back on, as long as the attunement remains.

In addition, you may add +1 to the armors AC for medium armor if your Dex bonus is 3 or higher

Scarlet Kiss

Starting at 1st level, you gain the power of dark, vile magic that is fueled by evil actions of deceit and cruelty. You may learn up to two profane magic spells, which count against your known Warlock spells.

During a short or long rest, you lose all profane points, and you may enter a dream-like state in which the thralls of Malcanthet teach you all about the art of seduction. You may roll 1d4 (starting with 5th level 2d4, with 9th level 3d4, with 14th level 4d4) to see how many profane points you have. You expend these points to use profane magic. Once per turn, you can gain 1 point by dealing damage with your profane touch. In order to use profane magic, you must be able to expend the required points, as described in the profane magical ability. You can never have more profane points than your Charisma modifier + your proficiency modifier.

You gain access to two new profane magic abilities at 5th, 7th and 9th class level.

1st: Profane Touch, Seductive Gaze
5th: Shrouded Deeds, Thwart Faith
7th: Tempting Glance, Vile Charm
9th: Unholy Demand, Stealing Touch

Profane magic uses Charisma as the spellcasting ability for attacks and saving throws.

 

Queen's Mask

Starting at 1st level, you sense the desires of those around you and can become their perfect mate.

As an action, you may polymorph into the ideal mate of a target creature, that is either in view or has been in view sometime in the last hour. The resulting creature can only be a small or medium-sized humanoid. If the creatures ideal mate would be a creature of different size or type, you become the next closest thing that fits the polymorph limitations.

Other than your size, your statistics are the same in this new form. Any equipment you are wearing or carrying is transformed in size, but doesn't change appearance.

If you die while in this new form, you revert to your true form.

The effect lasts for one hour. This ability can be used at will, without consuming a spell slot.

Pact of the Scarlet Lips

Randomly your body starts to change. You may grow either horns, wings or a tail for (Warlock Level^2) minutes. See effects below:

Horns: You have advantage on intimidation rolls and you can use a Bonus Action to deal 1d6+(Warlock level) bludgeoning damage and knock a creature prone.
Tail: You can use a Bonus Action to attempt to grapple a medium or smaller creature with advantage. As long as the creature is grappled by you, you can use a Bonus Action to deal 2d6+(Warlock level) psychic damage to the creature.
Wings: You gain a flying speed of 60ft.

You have no control over which transformation you go through at the beginning, but learn it over time. Before 3rd level, the effect may happen randomly.
Starting with 3rd level once per long rest you can use an action to force the transformation. 
Level 3-4: Roll a d6 every time to determine which transformation happens (1-2: Horns, 3-4: Tail, 5-6: Wings).
Level 5-6: Make a DC12 charisma check. On success, you can choose the transformation, on fail continue to roll a d6 as before.
Level 7-9: Make a DC12 charisma check with advantage. On success, you can choose your transformations, on fail continue to roll a d6 as before.
Level 10+: You have full control over your transformation.

At 6th level, you can pick two forms per transformation. At 10th level, you gain all three forms per transformation.

Devilish Proficiency

At 6th level, you realize you must embrace both your attractive qualities and your deviant cunning to achieve your aspirations. Whenever you make a Charisma skill check in which you are proficient, the die roll can never be lower than 5. This number gains +1 each level until it is 10.

Fiendish Diplomacy

Starting at 6th level, your thralldom reflects Malcanthet's recent allegiance with Hell and the succubi's role as advisor, consort, and tempter between fiends of the Lower Planes and mortals. Because of this, you are particularly adept at socializing and interacting with fiends. Whenever you roll a Charisma skill check in which you are interacting with a fiend that is not hostile, you roll at advantage.

Additionally, you gain proficiency in Persuasion and Deception. If you were already proficient in these skills, your proficiency bonus is doubled for any Persuasion and Deception skill checks.

Elluvia's Curse

/* needs a rework, especially limiting who can be targeted */
Starting at 10th level, through your touch you may cause a creature to seek you out until you utterly control them. Choose a humanoid creature that you embrace as an action that is either willing to be embraced or is incapacitated. Your target must roll a Wisdom saving throw or fall under Elluvia's Curse. Cursed targets are charmed by you and feel a euphoric sensation when first cursed, granting them advantage on their next attack, saving throw, or skill check. During the action they have advantage, they feel the same sensation as when you touched them and they see an enticing vision. Suddenly, the vision is gone and they are consumed with a desire of your embrace. They now have disadvantage on all attacks and ability checks until they return to you and you use this ability again, creating a cycle of bliss and torment. Upon the 3rd time you curse the same creature, they are then charmed and dominated by you as though a permanent dominate person spell had been cast on them. Unless the source of the damage is from you, damaging a cursed target does not grant them an additional saving throw to break free of this effect. Only a remove curse, greater restoration, or wish spell can break Elluvia's Curse.

You may use this ability a number of times equal to your Charisma modifier, regaining all expended uses after finishing a long rest.

For every creature you have dominated by Elluvia's Curse, you must expend 1 profane point whenever you finish a short or long rest up to your Charisma modifier. If you cannot expend the number of profane points required to maintain domination, the curse effect ends for a number of creatures equal to the difference. You choose which creatures are no longer dominated.

High Succubus Form

Starting at 14th level, the Queen's corruption has found its way to your heart and you are giving yourself to her completely. Your skin has turned a reddish-purple, demonic eyes claim temptation and terror. Your horns, tail and wings and their effects are now permanent. Instead of going through the demi-succubus transformation, you can use an action once per long rest to become a High Succubus for 1 minute.

The High Succubus form has the following benefits:

• Your movement speed is increased by 10, your flying speed is doubled
• Every time you deal damage, you can add an additional 2d10 psychic damage
• You have advantage on saving throws against spells that target you directly
• You are immune to all charming effects and spells that affect your mind
• Your charm spell is so powerful that you can tell charmed creatures to hurt themselves. They have to succeed on a DC10 charisma saving throw or will follow your order (e.g. stab yourself, jump off a cliff, cast fireball on yourself). The creature will only perform one self-harming action. If the target succeeds the saving throw it is immune to any self-harming commands coming from you in the future.

Once you revert back to your normal succubus form, you suffer one point of exhaustion.