Ravenloft has a special brand of villain that deserves a little extra attention: the recurring villain. I know, recurring villains are not unique to Ravenloft, but few settings have so many baddies that are actually designed in-game to recur. Many Ravenloft Darklords--and quite a few non-Darklords--have the "Undying Soul" special quality written right into their stat block. While the means of recurrence varies, these villains all spell unending pain to any bullheaded players who just want to kill things without getting into the drama. Using such a villain is one way of persuading players that, at least in the Land of Mists, sometimes the best you can do is stop things from getting worse.
With that in mind, here's a sample of the best recurring villains for your game:
1. Toben the Many--Any list of such villains has to start with the one who defines the category: Toben is the definition of recurrence. Arguably the most talked-about part of the old Van Richten's Guide to the Walking Dead, Toben stole the show in that book and has been a fan favorite ever since. His creepy eyes and grin are the stuff of nightmares, as is his matter-of-fact discussion of how much he enjoys collecting people.
Pluses: Toben is perhaps the most infinitely scalable villain, ever. Let low-level PC's meet just a few of his zombie collective, and get their chills with the challenge of making sure one doesn't get away to spread the plague. Higher level ones might face a whole village of hive-minded Tobens, with the challenge being reversed: can the PC's get away from Toben? For scholarly PC's, allow them the moment to match wits with a bound or incapacitated Toben host, like the Weathermay twins did. And don't forget: the Guide to the Walking Dead is an actual book in the Ravenloft setting. Giving PC's a chance to read the book is a great way to set up for their first visit from this particular "co-author."
Minuses: They say being a one-trick pony isn't bad if the trick is good enough. Toben really tests that idea. At the end of the day, the vast majority of his hosts are just creepy looking zombies, albeit slightly faster than ordinary ones. He just doesn't bring a lot to the table to enhance them with, except the single mind to coordinate them. In addition, PC's above 5th level who have met him twice will probably have developed some working strategies to minimize infection and maximize their attack value. Toben should learn to frustrate some of these strategies, but his resources to do so won't necessarily scale with the PC's gear. You can increase the number of hosts to greater and greater levels as PC's level up, but if you do this too much, your players may give up in frustration--it's like asking them to eradicate the common cold.
2. Harkon Lukas--Just like any such list must include Toben, Harkon also needs special recognition here. Sure, he's only one of many Darklords with the Undying Soul feature, but he's the only one to make it part of his battle strategy. Combined with his cursed berserking sword, Harkon's game of fighting to the death only to summon the sword to his new body and keep fighting has been a feature since the Black Box.
Pluses: Harkon is a good foe for teaching arrogant players humility. Even if the PC's never learn it in-game, let the players learn the secret of his immortality: his soul goes to the nearest dire wolf within his domain borders. What are they going to do to stop him for good--kill every dire wolf in Kartakass AND prevent more from crossing the border from Barovia and elsewhere? Prevent the Mists from dropping just one more dire wolf off somewhere in the middle of the woods? Short of starting a wolf-centered plague, even epic PC's would be hard pressed to kill Harkon permanently.
Minuses: Never forget he's a Darklord--and don't let your players forget, either. There's nothing worse than having Harkon come after high-level PC's who have already killed him so many times that they start laughing about new ways to kill him. No DL is supposed to be a joke to face, and Harkon's continual return should not be a laughing matter. If the PC's are too powerful for him to use his wandering-bard schtick, introduce them to his political prowess as Meistersinger of Skald. Whether he turns public opinion against them, or works through quislings, or works his wiles in female form, be ready to show them that Harkon Lukas is not JUST an undying soul. He's also one of the blackest.
3. Ejric Spellbender--while any odem (or a ghost with Malevolence) might fit the bill, Ejric was the prototype for possession horror in Ravenloft. Way back in the Book of Crypts, Ejric's takeover of a PC was the result of a magic jar spell gone wrong, but the Fraternity of Shadows has since upgraded him to an official odem in their Nocturnal Sea Gazetteer. He's still limited to taking over people who are sleeping or unconscious, but his frustrations with that limitation might get him traveling to the mainland in search of help, so don't think he's just limited to Armeikos or even Liffe.
Pluses: Ejrik's use of sleeping hosts is sure to give players some moral dilemmas--just have him take over a child in a dangerous situation where expelling him might endanger the host. Subdual damage or sleep magic doesn't expel him, because his host is already unconscious. Charm and hold magic doesn't work on him either, because he's undead. If the PC's manage to ensnare his current body, he can just leave and find another. In a port town like Armeikos, there ought to always be a few drunks passed out in the many dives down by the docks, day or night. If the PC's have the ability to harm him in incorporeal form, or prevent him from controlling a host, he can just hunker down inside the host and wait it out. After all, he's got all eternity to wait.
Minuses: Once PC's have figured out how Ejrik works, his days are numbered. With the right spells and/or enough holy water, a group above 5th level might manage to pin him down and destroy him for good. He's best used against a lower level party, in combination with other threats, and/or from a position of surprise, so they are not packing the right heat to destroy him. If they have faced off with him at low levels, have him run away for a while and lick his wounds, waiting until they are facing another threat before he shows up for the tag-team. On the other hand, restore his full spellcasting abilities--he was originally a 10th level wizard, why limit him to one spell?--and see if that doesn't give them a run for their money.
4. Jacqueline Montarri--A true gem of the 2nd Edition Monstrous Compendium II, Montarri not only set the bar high for the horrible things that could be done with a Vistani curse, but she also laid the groundwork for the truly time-warping powers of Madame Eva and the Zarova Vistani. Nor did Montarri wind up in the retcon dustbin with the arrival of Gaz1. On the contrary, she got an upgrade to the head of Red Vardo Traders and a spot with the big boys in the appendix. But what can you do with her?
Pluses: Of all the villains on this list, Jacqueline Montarri is perhaps best suited to appear in many roles besides that of adversary. Red Vardo Traders might serve as a background piece to give Barovia flavor, or as transportation if the PC's are in need of a lift. They might offer black market goods for sale, or end up as rivals with the PC's in some race for a magic trinket. This gives opportunities to hear about their mysterious leader before meeting her face to face...to face.
Minuses: If you are going to use Montarri as a villain, there's little point unless the PC's hear her story. Without that, she's just another crazy mad killer with her signature weapon collecting trophies from each victim. Combine this with a lengthy intro by RVT as above AND the multiple reappearances to illustrate her immortality, and she runs the risk of taking over your campaign. While I admire a go-big-or-go-home approach, this may be too much for even an experienced DM to manage. Unless you really want to build the entire campaign around the search for her head, you might be better off just letting her cameo for the sake of experienced players' geeking out.
5. Lucre the Coin Golem--Hand it to Stan! to remind us that the recurring villain need not be living or undead, but might actually be a construct, if that construct was composed of something as ubiquitous as gold coins. This contribution to the Children of the Night: Created was among the most common favorites, for obvious reasons.
Pluses: Are your PC's poor? Run them through the original Heart of Gold adventure. They will have a lot of money, if they survive. On the other hand, if it's too much, or if your players getting greedy, have Lucre return and run off with all the gold. Even as PC's learn to diversify into gems, jewelry and other things Lucre can't animate, the golem can learn about buying and selling, and may start selling non-gold items to amass coin hoards for himself. Once PC's learn about his thirteen special coins, the golem will mark them for death. By the time they melt their first coin, at least two others should be scattered beyond the Core, making Lucre one of the hardest non-Darklords to kill, even when you know exactly what to do.
Minuses: Lucre is a great villain to end this list with, because he embodies the weaknesses of many of the others. Like Montarri, Lucre's backstory demands to be told, and his reappearances stand to derail any other plot you may have planned. Like Ejrik, he is vulnerable in his isolated state--a single coin is helpless--and might be destroyed by mid-level spells. Like Toben, his recurrence is his best trick, and players may scale up much better than he does, as they learn what works on him. He does not share Harkon's weakness, which is perhaps his greatest failing: despite his immortality, Lucre lacks the versatility and the pathos to be a Darklord.
I am become death, destroyer of worlds.