The transformation into a death knight has decayed your skin, leaving only a skeleton held together by black fog. Your eyes glow a terrifying color. Your voice gains a resonating and booming quality, along with a whispery undertone. You highly prefer armor on at all times, and you begin to seek power by force. Your alignment switches to Lawful Evil, and you gain the flaw: "I will hold dominion over the weak."
Death Knight Spellcasting. You are a 19th-level spellcaster. You lose the ability to use healing magic. Your spellcasting ability for these spells are Charism, and you have the following paladin spells prepared:
1st level (4 slots): command, compelled duel, searing smite
2nd level (3 slots): hold person, magic weapon
3rd level (3 slots): dispel magic, elemental weapon
4th level (3 slots): banishment, staggering smite
5th level (2 slots): destructive wave (necrotic)
Power of Undeath. Your Strength and Constitution scores are raised to 20 if they weren't already higher. Your Charisma score is raised to 18 as well.
Rejuvenation. If destroyed, your body will rejuvenate with all gear held on your body 5ft. from your transformation site in 1d10 days, if it is block, regenerate at the highest inhabitable level at that point. If you can be convinces to seek atonement, you will lose this ability and fade away.
Magic Resistance. You have advantage on saving throws against spells and other magical effects.
Undead Nature. You don't require air, food, drink, or sleep.
Marshal Undead. Unless you are incapacitated, you and undead creatures of your choice within 60 feet have advantage against saving throws that turn undead.
Damage Immunities. Necrotic, poison.
Darkvision. 120 feet.
A death knight can be trapped or sent to a different place to disable them for some time, but the only way to truly destroy them is to convince them to atone for their sins and help them to carry out those actions. A death knight player must make Wisdom saving throws against other players that are trying to convince them.