Undead mage

An undead mage is someone who is half way between the material plane and one of the many afterlives of the d&d multiverse. Because of this they can draw power from the space between life and death in order to use magic. The reason someone becomes an undead mage can be anything, maybe they’re the result of a failed resurrection or came into contact with something magical that almost killed them but not quite, maybe they have the same backstory as a revenant or ghost where they were literally too angry to die. Regardless, being undead means you are cold to the touch and your skin is pale and flaky. You might want to customise your character to fit the afterlife that they are partially resting in. If its the abyss, hades or the nine hell’s you could have red patches on your skin or a single, small horn on your forehead. If it’s any of the upper planes you could have glowing eyes, or perhaps your skin is still pale but instead of flaky it’s smooth and clear like a vampire.

Half life

At 1st you are partially undead, granting you the following abilities 

• you do not feel pain 
• you are considered an undead instead of a humanoid 
• you have advantage on death saving throws 
• you do not need to eat, drink or breathe and instead of sleeping you enter a state that functions exactly like an elf’s trance. In this state your soul wanders through the afterlife you would go to if you died and your body is indistinguishable from a newly dead corpse.
• while you are making death saves, you remain conscious and can use all of the senses. The only things you can do are talk and make slight movements like pointing your fingers.

Undead magic

Also at level 1 you gain the spell false life. At 5th level you will also gain the spell animate dead 

Soul projection

your soul is partially dislodged from your body Allowing you to project yourself in the form of an incorporeal spirit that can possess corpses. As a bonus action you can project your soul out of your body. When you do so your body becomes limp and for all intents and purposes is dead. You still have the same amount of hp as when you used this ability and you can be damaged but not healed. If you’re body drops to 0 hp you return to your body and have to start rolling death saves. Any spell that raises the dead being cast on your body (eg true resurrection, animate dead, revivify ect) will result in your immediately returning to your body. While in spirit form you retain your place in initiative order but can move through objects, cannot be attacked or be effected by anything. The only things you can do in this form is move with a fly speed of 40ft and use your action to either return to your body instantly or try to possess a corpse/skeleton or an undead creature that has physical form and is cr 1/2 or less. If you chose a corpse you retain your initiative order but now in the form of a zombie or skeleton. If you chose un undead creature the creature must make a charisma save against your spell save dc. On a success you waste your action but on a failure you gain control of that creature. Once you have possessed a body the only way to leave it is to return to your body as a free action, if your new form dies you immediately and involuntarily return to your body. You can use this ability a number of times equal to your proficiency bonus.

Detachable extremities

Thanks to your supernatural anatomy, at 14th level your limbs can be detached and still function on their own. As an action you can remove one of your extremities. If you remove your head you suffer the effects of the deafened and blind conditions but you can see and hear through your head wherever you put it. If you remove one of your arms you can only hold one thing at a time you have disadvantage on strength checks and can’t use two handed weapons, if you remove both arms you cant hold anything and you can’t cast spells that have somatic components. If you remove your leg your walking speed is halved and you have disadvantage on dexterity saving throws and acrobatics checks, if you remove both legs your speed becomes 0 and you automatically fail dex saves and acrobatics checks. Removing either an arm or a legs summons a limb in an unoccupied space within 5 feet of your. Limbs have their own stat block linkedhere (certain abilities can only be used if the limb is an arm, these abilities will be detailed in the stat block). If you soul project all your detached limbs become limp until you return to your body. If the limb drops to 0 hp they can be reattached and will function like normal but if they are detached again they will not move. Your head has the same amount of hp as the limbs and if it dies you must start making death saves. If an attack that deals slashing or bludgeoning damage lands a crit on you you must make a constitution saving throw. On a failure you roll 1d6 to determine which body part gets unintentionally detached.

1. Head
2. left arm
3. right arm
4. left leg
5. right leg
6. nothing  

Wellspring of souls

At 18th level you can draw souls from the afterlife in order to regain magical power. But taping into this power is risky. As a bonus action on your turn you can choose to try and draw souls from the after life, you can choose how many souls you draw at once. When you do so make a charisma saving throw, on a success you regain a number of sorcery points equal to 1d4 times the amount of souls you drew. But on a failure a number of specters equal to how many souls you drew appear in the closest unoccupied spaces to you, and they are all hostile to you and anyone who tries to stop them from killing you.