Hullo hullo! I've been lurking here on this thread since the start and have immensely enjoyed every monster review and the discussions that follow. With the inexplicable absence of our beloved OP it will be my honor to carry on the torch with this post about a dude carrying a torch!

Death Knight

A classic villain and antithesis to the noble Paladin, this fallen champion of good is about what you would expect. Undead, bane of all life, tortured soul, etc.

Art

I'm not a fan. The things that stick out most are the aforementioned torch, which serves no thematic purpose, and the goofy cylindrical helm. The only things that speak to the evil incarnation of undeath we are supposedly being presented are the obligatory red glowing eyes and the skeletal hand that seems to me to be a last minute edit when one guy realized there was no apparent evidence of the Death part of Death Knight. There ought to be more dynamism and drama. Covering the face was a missed opportunity. They should have displayed a nuanced expression showing the dichotomy of rage against the living and self-loathing at his own wretched state. Not a terrible picture objectively, it just doesn't do it for me.

Purpose and Tactics

High AC and a big old pile of hit points makes this a tough SOB to take down. Add in magic and turn resistance, a reaction AC boost and literal immortality gives you the makings of a perfect recurring nemesis. The turn resistance can be granted to nearby undead, but other than that there isn't much to highlight the Death Knight as a leader of undead. You should run it with some allies to even out the advantage of action economy since it doesn't get Lair Actions (or you can give it LA's from another source like the Lich), but they don't necessarily need to be undead mooks.

Fluff

A fallen Paladin cursed to eternity as an undead until it repents. A fine trope that kinda falls apart with the inclusion of the Oathbreaker Paladin. This is less a tragic, torturous fate and more of an ultimate reward for a life lived sadistically. Since by definition every Oathbreaker that dies will become a Death Knight, and Death Knights can never be fully destroyed until they repent or are "redeemed" by some outside force, you have the makings of a bunch of Saurons minus the One Achilles Heel.

That being said, the fluff is fun. The story of Lord Soth is great; a little too short to give a clear understanding of how a noble Paladin could have become so overcome with lust as to have his wife murdered so he could cheat in his own bed, but compelling nonetheless. With a little more depth of detail you can really weave an engrossing tale of honor, temptation and tragedy into your campaign with an epic battle as payoff in the late game.

Hooks

No matter how many times they destroy it, this damn villain just keeps catching up to the group and getting in their way! How us an adventurer supposed to enjoy the spoils of grave robbing and corpse looting with a stupid, unkillable punching bag always crashing the party? It's not like there are other ways to solve a problem than violence...

Throughout the campaign your Paladin player has been utterly careless about upholding the Oath he ostensibly swore, and now the consequences of his borderline murderhobo-ish ways have finally gotten him killed. He rolls a new character and the story rolls on, but a new threat has appeared in the world to hinder the party's goals. I wonder who that could be?

A valiant knight once bravely descended into a cursed city to slay a great, rising darkness and prevailed at the cost of his life. Years later, the darkness is rising once more, and those few who enter the city and return alive tell of legions of undead led by a shadowy figure bearing an all too familiar sword.

Verdict

Predictable but well executed. Whether you build them up as the Big Bad or the Dragon (see TV Tropes), the Death Knight will serve as a reliable source of epic combat encounters with a twist; swords and spells will never end the threat, but merely delay it. Roleplay is the only way to win, and that's a refreshing change of pace.