Unfinished

Not all undead return against their will. Most return do to some arcane loophole or the actions of a dickish wizard. But a few return by sheer force of will, and four small words: "I am not done."

You are not done.

As a result of your not-done-ishness, you gain the following features. 

Undead. Just because you are not done does not mean you come back to life. Your creature type is now undead, and you are resistant to poison and necrotic damage. In addition, you no longer need to eat, sleep, or breathe. 

Revenance. You cannot truly die until you are done. You have advantage on death saving throws. Moreover, if you die before completion, you come return, with all hit points, and all conditions and diseases cleared, in the nearest unoccupied space from where your body was committed to the earth. If you were buried, you dig yourself up. If cremation occured, the ashes reconstitutes your form. In all cases, your soul returns to your body, and if that body no longer exists, the magic creates a new one. As a result of your semi-permanent undeath, you cannot be revived. 

Because you don't truly resurrect, you still rot. When your body fully decomposes, the world recognizes your soul's need for a home, and reconstitutes your body, as if you had just died. 

Strength in Death. Your attacks against creatures that are alive (so, not undead and elementals) have advantage. if you can cast necromancy spells, your range for those spells are doubled, and you can reroll damage rolls of 1 or 2 as many times as you want. if the spell is cast as a ritual, it instead takes the normal casting time. This bonus only applies to necromancy spells.