The way of the shadow phoenix is reached when you have died, but have been granted life again at the cost of sharing your body with another host.

Phoenix Ancient Weaponry

At level 3 you learn how access a small portion of the astral plane, allowing you to store one weapon in it at a time. This weapon can only be changed out in one way, in which it has to be a magical weapon you can attune to, but instead, you can spend your attuning replacing the weapons. You can summon this weapon at any time using a bonus action for both storing and summoning. Whatever weapon you store, you are automatically proficient with, and it gains a shadowy aura attached to the weapon, dealing extra necrotic damage equal to your damage die. The aura stuns your opponent briefly, allowing you to do an unarmed strike after you hit. 

 

Shadow Phoenix Techniques

Starting at level 6 you constantly dream in the astral plane now, which has allowed you to learn more techniques using ki points.

Phoenix Regeneration- At the cost of 3 ki points, you can heal yourself equal to your martial arts die, plus your level.

Phoenix Aura- At the cost of 3 ki points, you can cover yourself in a shadowy aura for 1 minute, when you are physically hit, you can have the aura strike the attacker for half of the damage done in necrotic damage, 1 extra ki point can be used to hit the attacker if it was a ranged or spell attack. The aura disappears as soon as the ability is used.

Burning Fist- At the cost of 2 ki points, you can take an extra unarmed strike dealing necrotic damage, the target must make a con save equal to your wisdom save dc, if they fail, they are magically blinded by the shadow aura for 1 minute, or until they pass the dc, they re-roll at the start of each of their turn.

Phoenix Form.

At level 11 you learn how to allow the phoenix to partially take over your body for 1 minute clouding both your body and mind in shadows, taking the form of a phoenix, giving you a flying speed equal to your speed including your unarmored movement. Whenever you make an attack, you gain two martial arts dice of necrotic damage. When you are hit in this form, you must pass a wisdom saving throw of ten, every time you pass, you add one to the dc until you fail, when you fail, you lose all control to the phoenix, in which it will go on a rampage forcing you to attack the closest person to you, friend or foe. You must take a long rest before you can do this again.

Phoenix Revival

Starting at level 17, when you reach 0 hit points, you can stabilize yourself to 1 hit point while also unleashing a shadowy aura within 30 ft, hurting all of those you don't consider a friend with 10d10 necrotic damage. You can add an extra 1d10 at the cost of 5 ki points. You must take a long rest before you can use this ability again.