The Knight of Solamnia

The infamous Death Knight, Lord Soren Soth, once one of the most noble knights of Solamnia, began his fall in grace with an act of heroism, saving an elf named Isolde from an ogre. Soth and Isolde fell in love, but Soth was already married. He had a servant dispose of his wife and was charged with murder, but was rescued by his fellow knights and fled with isolde into Dargaard's Keep. Since then, Soth began to grew more bitter. It was said that he started mistreating his wife, both verbally and physically. One day, the god Paladine gave him a chance of redemption, he gave him the task to stop the "Kingpriest" of Ishtar, whom labeled himself as a prophet of all gods, and everyone called a mad man.

Soth Agreed, but on the road he met with some of the elves who were rescued along his wife, and poisoned Soth's mind with wispers of her wife cheating on him. Soth forgot about his quest, and confronted Isolde. By the time he arrived Daargard keep, the gods sent a Cataclysm, to swipe the heresy that was the Kingpriest and his followers. As fire started consuming the castle, Isolde asked Soth to save his child and flee the castle, but Soth denied them both, and with her last breath, she cursed the now Death Knight for one thousand lifetimes.

Now Lord Soth remains as the lord of Dargaar keep, few are the fools who would try go in, even the Vampire lord Strahd Von Zarovich respected him both as a fighter and a tactician

"Soth lashed out at the vampire lord, but Strahd caught his wrist. The vampire held him fast, and the two dead men locked gazes

-'No one has the power to challenge me here' The vampire smiled viciously.'All who dwell in Barovia serve me, as you will soon learn, Lord Soth'."- Knight of the black rose.

Lord Soth's Lair

Dargaard's keep is a place of madness, with a long history of treason, murder and necrotic energy. On the outsides of the castle, there is no sign of life. All the undead move attracted to the keep like flies to a lantern. There's always at least one Death Knight watching the perimeter, wearing the armor of a knight of Solamnia. The perimeter around the keep is clouded with a black mist and the air is foul, rejecting all unwanted guests.

The sound of shrieking and screaming can be heard from the keep, unless Lord Soth isn't there, which in that case thirteen Banshees roam the place

Lair actions

On initiative count 20 (loosing initiative ties), Lord Soth takes a lair action to cause one of the following effects; Lord Soth can't use the same effect two rounds in a row:

-The doors are close shut, and cannot be open by anything short of a knock spell or an Athletics check (DC 18)

-Lord Soth summons 1d4 -1 Death Knights with the reduced threat rules applied to them (Tales of the Yawning portal page 113).

-One Banshee Appears and uses her Wail action, and every creature that hears (even Lord Soth) must make a DC 15 constitution saving throw.

-Lord Soth Summons 3d6 Revenants

Lord Soth as a BBEG

Lord Soth is one of the most tragic and interesting characters in Dungeons and Dragons, much like Strahd, an adventure that relies around him should be rich in lore, and should give the characters the chance to meet him indirectly, because unlike the count, Soth is a killing machine, and has no regard for other lives, if he meet the characters, there should be a good reason why they are still breathing.

You can, for example, introduce him in the adventure Curse of strahd As an NPC for later in your campaign, once they defeated Strahd, he would not attack the characters since it's Strahd's domain and won't take a role in the story, he's just there with his own business (maybe a trip to the amber temple or a sit down with the vampire lord himself)

Lord Soth Equipment

Lord Soth is equipped with the following magic items

-Plate armor +3, which is inmune to the effects of Heat metal

-Ring of spell storing with the following modifications:

Instead of storing a spell, Lord Soth wields this ring to trap the soul of a victim which level is equal or lower than 10, it acts with the effects of the spell imprisionment, breaking the ring ends this effect. While Imprisioned this way, Soth can access the soul, and put it in a dead body, creating a creature with the stats of a Revenant, it remember who he is and how his life ended, but must obey Lord Soth at any cost. If the creature dies, it comes back to the ring, and Lord Soth cannot use this property again for 1d4 years.

Runeblade (Greatsword, Legendary)

Runeblade it's a lawful evil sentinent weapon, seeking to destroy Lord Soth, but unaviable to unattune from his actual master. It can cast the spell Sending once per year, targetting only a creature with the intentions of destroying Soth

You gain a +3 bonus to attack and damage rolls made with this magic weapon. Additionaly, you gain access to the following feature.

The true ruler(1/Long rest) As an action, you can activate the Runeblade to add 24(6d6) Fire damage to your weapon attacks made with this weapon, and the first creature hit in each turn must make a Charisma saving Throw (DC 18) or become frightened for 1 minute. While this is active, you cannot regain hitpoints. It lasts 1 minute and you gain an exhaustion level.