Requires: The at least two levels in Wizard (Necromancer) or Cleric (Domains: Death or Grave).
You undergo a magical ritual called "Crucimigration" during which Necromantic magic infuses your mortal form and sustains you, at the DM's option this may require certain research, items or material components to complete but the base version would entail meditating in a coffin or tomb for at least 8 hours. Your body becomes unusually thin and your flesh is taught over your bones giving you a freakishly gaunt appearance.
Once the ritual is complete you no longer need to sleep, eat or drink, however if you are a spell caster, you still need a Long Rest to prepare your spells or pray to your God for spells.
You are also immune to magical sleep
You also no longer age but can be aged by magical affects
You can eat if you wish to but you gain no pleasure from doing so, all food and drink tastes like ash. Should the food/drink be poisoned or diseased you can still be affected by those conditions.
You now discern as Undead to any spells or abilities that would detect them, although you cannot be turned/destroyed nor do you take extra damage by spells or abilities such as Channel Divinity as this feat does not actually make you Undead. There is however one major drawback, the necromantic energies infusing you inhibit healing magic, you only receive 50% of any healing form magical means.