Pact of the Fused Flame

Through performing a forbidden ancient ritual, your soul has been split into pieces and is able to fuse with fiends. the cost of fusing your mortal form with the fiend is the slice of soul you promised them, in combat you are able to call upon them in order to assist you in battle, briefly taking on their form to do with as you see fit, however fiends are fickle, if they feel you are taking advantage of their "generosity", they will abandon their pact or demand a bigger cut of your soul. 

 Warlocks of the fused flame are easily identifiable, usually bearing some kind of mark, sigil, scar or brand for each of the fiends they have fused with to show they they have been "claimed", while the nature of this mark is negotiable with the fiend, they must always bare something of that nature. 

in some cases, the fused host may even take on certain traits of the fiends he or she has chosen to fuse with. a warlock fused with an imp may have their skin become a redish hue, or have horns grow from their head if they do not already have them, alternatively if the host has a tail, it may take on the appearance of a scorpid. 

Fiend Form

When you choose this pact, You can use your action to Call upon the form of a fiend that you have fused with. You can use this feature a number of times equal to your Charisma modifier. You regain expended uses when you finish a short or long rest.

Your warlock level determines the Fiends that you can fuse with, you gain a soul slot for each warlock level you have. for each CR level of the fiend you intend to fuse with, you must promise 2 soul slots to them. 

You can stay in a fiend form for a number of hours equal to half your Warlock level (rounded down). You then revert to your normal form unless you expend another use of this feature. You can revert to your normal form earlier by using a bonus action on your turn. You automatically revert if you fall unconscious, drop to 0 hit points, or die.

While you are transformed, the following rules apply:

• Your game statistics are replaced by the statistics of the fiend. You retain all of your skill and saving throw proficiencies, in addition to gaining those of the creature. If the creature has the same proficiency as you and the bonus in its stat block is higher than yours, use the creature’s bonus instead of yours. If the creature has any legendary or lair actions, you can’t use them.
• When you transform, you assume the Fiend’s hit points and Hit Dice. When you revert to your normal form, you return to the number of hit points you had before you transformed. However, if you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form. For example, if you take 10 damage in Fiend form and have only 1 hit point left, you revert and take 9 damage. As long as the excess damage doesn’t reduce your normal form to 0 hit points, you aren’t knocked unconscious.
• You can’t cast spells unless the fiend you are transformed into has the ability to cast spells, in which case you may cast spells as they would. Transforming doesn’t break your concentration on a spell you’ve already cast, or prevent you from taking actions that are part of a spell, such as call lightning, that you’ve already cast.
• You retain the benefit of any features from your class, race, or other source and can use them if the new form is physically capable of doing so. However, you can’t use any of your special senses, such as darkvision, unless your new form also has that sense.
• You choose whether your equipment falls to the ground in your space, merges into your new form, or is worn by it. Worn equipment functions as normal, but the DM decides whether it is practical for the new form to wear a piece of equipment, based on the fiend’s shape and size. Your equipment doesn’t change size or shape to match the new form, and any equipment that the new form can’t wear must either fall to the ground or merge with it. Equipment that merges with the form has no effect until you leave the form.
• If you drop to 0 hit points while assuming the form of a Fiend, you may not call upon that form again until you have completed a long rest.

Primeval strikes

Starting at 6th level, any attacks you make in Fiend form count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.

Sacrificial vigour

you are able to sacrificed the life force of a fused fiend in order to continue using your current form. at 10th level when dropping to 0 hit points in fiend form, you may sacrifice another fused fiend and roll their hit die, regaining hit points equal to the amount rolled. once this feature has been used you may not use it again until you finish a long rest.

you may not assume the form of the sacrificed fiend for 7 days.

Unstable fusion

Powerful fiends are more willing to fuse with you, your soul is worth more than that of your typical adventurer however more powerful forms are unstable and hard to maintain.

Starting at 14th level, you may pay for fiends using 1 Soul slot per CR level. Any form you assume that is higher than CR10 will cause you to suffer a level of exhaustion when reverting back to your true form. if you are reduced to 0 hit points while assuming the form of said fiend, you suffer 2 levels of exhaustion instead.

a fiend higher than CR10 takes longer to regenerate when destroyed, if a fiend is reduced to 0 hit point, it takes an additional day to recover for each CR over 10.