Ever heard of Faust?

On the verge of death, an eldritch voice speaks to your mind. It offers you new life, and lends you a portion of its power to defeat your enemies. In return, it simply asks for a favour to be named later.

The creature restores you and your abilities to full, as if you had taken a long and peaceful rest. All conditions, levels of exhaustion and other negative effects upon you cease, and you awake to find yourself stood up. Your health returns to full, and your abilities, hit die, magic items and any other special features you have recharge to maximum. Equally, for 1 minute after you accept the bargain, all attacks against you have disadvantage, and you have advantage on all saving throws, skill checks and attack rolls. During this minute, you glow faintly and if another character attempts an investigation OR perception check (DC20) focusing on you and the glow, they can sense the creatures presence (but not its form, nature or alignment).  

You also learn two cantrips of your choice from the warlock's spell list.
In addition, choose one 1st-level spell from the warlock's spell list. You learn that spell and can cast it at 8th level.
Using this feat you can cast the spell a number times equal to your PB, with uses reseting after a long rest.

Charisma is your spellcasting ability for these spells.

At the DM's discretion, the creature will call upon you to repay the favour. It will tell you to fulfil one goal it desires (such as 'Kill the King', 'Destroy the Church', 'Bring me the Amulet', etc.) but achieving that goal that may involve any number of smaller tasks, ranging from the very small and precise, to the very large and complicated. If you are willing to do what is asked, you act as normal until the bargain is complete. If you are unwilling, or at any stage you become unwilling, to do what is asked of you, the creature causes you to take 3D8+8 psychic damage (ignoring resistance and immunity), and casts the 8th level variant of Dominate Person (DC20) in order to force the completion of the bargain. While under the spell, the creature cares not for your wellbeing, and only for the completion of the task. If you successfully resist at any point, the creature repeats this attempt each hour on the hour, unless you are in an Antimagic Field.

Once the bargain is complete and the task fulfilled, the creatures power abandons you, but a portion of it remains behind like an echo - replacing this feat with the standard Magic Initatie: Warlock feat with the same spells chosen.

If you die at any point with this feat, the creature claims your soul as payment and traps it in its realm, systamatically tourturing it for failing to fulfil the bargain until the end of time. This effect overrides similar magic or items with a soul storing feature such as the Ring of Mind Shielding. At the DM's discretion, they may allow a party to discover and travel to the creatures realm, challenging it for the release of the soul.