Undying Mocker

"Kill me once? Shame on me. Kill me twice? Shame. On. You." -Unknown

Something both fey and shadowy possess you, allowing you to remain standing even long after any mortal should be long dead, a living mockery to your enemy. You gain the following features:

• The amount of damage required to instantly kill you is increased by an amount equal to your level, and by 1 more every time you gain a level once you have this feat.
• When you are reduced to 0 hit points but not killed out right, you can use this reaction to remain conscious as you pass into a state that blurs the lines between life, death, and undeath. You must still make death saving throws while in this state, and taking damage results in a failed death saving throw. You can remain in this state for 1d6 minutes, and, while in it, if the damage that reduced you to 0 hit points was dealt to you by a creature, you have advantage on attack rolls against that creature. However, a side effect of this state is that it actively cancels magical healing around you. You cannot cast a spell that restores hit points, and all spells that restore hit points cast within 20-feet of you restore only half the amount of health they would normally heal, unless they are from the necromancy school. You can use this reaction a number of times equal to half your proficiency bonus, and regain expended uses at the end of a long rest, although you must succeed on a DC 15 Constitution saving throw every time you use it beyond the first or suffer one level of exhaustion.