The Great Phoenix

Your patron is a great and noble Lord or Lady of the Elemental Plane of Fire: An ancient Phoenix. It is a creature that has seen gods come and go, and to whom a life-age of the Earth is as a single day. These beings are viciously protective of life and seek to burn out evil, especially those evils that would attempt to cheat the sacred cycle of life, death, and rebirth. A Great Phoenix will often send lesser Phoenix into the world to accomplish these ends. Few Phoenix are powerful enough to forge pacts with mortal races, but those that are draw their power from the Fires of Creation itself.

Expanded Spell List

The Great Phoenix lets you choose from an expanded spell list. The following spells are added to the warlock spell list for you:

1st Level: cure wounds, faerie fire

2nd level: Aganazzar's scorcher, heat metal

3rd level: beacon of hope, daylight

4th level: fire shield, guardian of faith

5th level: conjure elemental, flame strike

Firebird's Fury

Starting at 1st level, your patron has ignited your soul with a mote of its own eternal life force and can release the fires of your patron during battle. You learn the create bonfire cantrip, and it does not count against your list of cantrips known. As an action, you wreathe yourself in the radiant fires of your patron. For 1 minute,  when a creature you can see within 60 feet deals damage to you or a friendly creature, you can use your reaction to force the attacker to make a Dexterity saving throw. It takes 2d10 + your Warlock level in fire damage on a failed save, or half as much damage on a successful one.

Once you use this feature, you can't use it again until you finish a short or long rest.

Spark of Rebirth

Starting at 6th level, the fiery energy within you grows restless and vengeful. In the face of defeat, it surges outward to preserve you in a fiery roar. If you are reduced to 0 hit points, you can draw upon the phoenix's spark. You are instead reduced to 1 hit point, and hostile creatures within 10 feet of you takes fire or radiant damage equal to your warlock level + your Charisma modifier.
Once you use this feature, you can't use it again until you finish a long rest.

Elemental Wings

Starting at 10th level, your patron has revealed the ancient Fires of Creation to you and taught you secrets about them that not even Angels know. Your soul is now more Phoenix than mortal. You gain the ability to summon wings formed by hundreds of long, fiery feathers. When these wings are active, you gain a flying speed of 60 feet. You can create these wings as a bonus action on your turn, they last up to a number of hours equal to half your warlock level (per long rest) and can be summoned and dismissed at will.

Additionally, as an action, you can wrap yourself or another friendly creature in your wings to channel the fiery energy of Creation into their soul. The target regains 10d10 hitpoints.

True Rebirth

You have now gained enough favor with your patron that they have allowed you access to the Fires of Creation that keep them alive. When you fail your third death saving throw or are subjected to an effect that would kill you instantly (and you haven't made use of your Spark of Rebirth), your body bursts into flames and is reduced to ashes leaving behind any items or equipment on your person. After 24 hours, your body is reborn in the same form it was destroyed in (i.e. same race, same memories) but you return completely naked and in a random location on the plane of existence you died on. You also gain the effects of a long rest.