I ran CoS for two groups simultaneously and enjoyed my time doing both. That being said... running it has impacted how I feel about the way the adventure book is structured and set up, and were I to run it again, I think I would make some structural changes to the adventure to increase the tension.
Having run it, the biggest issue I discovered is that having the visit to Madam Eva occur so soon after the characters' arrival changed the atmosphere for the players. Receiving word about all these McGuffins that would allow them to destroy the Lord and escape the land immediately changed how they saw and experienced Barovia. They were now like participants in a
Saw movie, where there were bound by puzzles that had solutions and which kept them motivated to move forward. Thus the suffocation of the land's atmosphere never affected them that much. There's less depression and malaise when you have definitive direction and reason to constantly push forward. You don't notice the horrors around you if you just have tunnel-vision towards the things you need to solve your puzzle.
Now it's obvious why the adventure is set up this way... it mirrors the original I9 module where there's much less exploration. It's get in, get told how to get out, find the methods to get out, and then get out. And meeting Madam Eva almost immediately upon arrival was a part of that. But for this longer and more expansive campaign... I think the adventure might be better served by NOT giving out the solution so soon after arriving. DON'T have the party get told immediately after arriving in the Village to go find Madam Eva and have their fortune told. Instead... force the players to live within the Valley for a while and experience what the claustrophobia and suffocation is like. Give Strahd more opportunities to toy and play with the party
before they are told about or given the ways to defeat him. Really put the screws to the NPCs in the various villages of the Valley, especially after the party befriends some of them. Make things more hopeless for the party before learning about the Sunsword or the Holy Symbol of Ravenkind and then giving them hope. Thus you build Strahd up as a true and seemingly invincible villain before having the party discover the secret into possibly overcoming him.
And having thought about it further... I suspect there's also one other thing I think I'd need to do to truly make this adventure into the horror play that it truly should be...
...give Strahd von Zarovich Level/Experience Point Draining.
I think all of us know that the actual 5E game is not designed to be truly balanced for the larger gaming populace. Instead, it is designed and balanced to work best for the newest of the new players-- 4 PCs using Basic Rules. If you have a Champion, Thief, Evoker, and Life Cleric and that's it... the monsters in the Basic Rules can be credible threats at the levels / CRs as listed. But if you add any player-facing material from the PHB beyond that, they begin making incremental changes towards being easier. Barbarians getting Resistance when they rage, Paladins gaining immunity to diseases and removing poisons while Smiting everything under the sun, Rangers making exploration trivial, Bards giving bonuses to people left and right, etc. etc. Couple that with adding in more PCs above 4 and suddenly we discover that the monsters in the MM don't really hold up as credible threats anymore because there's too many hit points to fight through and too much healing to keep PCs from being killed. And when you start LEVELING all these PCs with all their bizarre class combinations... pretty soon there's nothing they can't start steamrolling unless you really force yourself to re-work all the monster stat blocks. After a certain point, HP damage is not high enough to seen as much of a threat an longer.
But what COULD be seen as a true threat to jaded D&D players?
Not being able to level up. Being stuck at 3rd level trying to deal with the horrors of the realm over and over and over again. What better way to truly make Strahd the scary villain he is than to have him show up on the scene just as the PCs are all ready to gain just enough XP to level up, only for him to attack them and drain them of XP to send them spiraling back down to the bottom of the level? That might be the only credible way to truly freak players out-- Strahd not letting them become more powerful. Yeah, it does run the risk of alienating them if it's done unwisely... but I think if you leave it merely as a Strahd ability... then at least the players could feel like they have a fighting chance.
After all... what's the reason for Strahd to attack the PCs otherwise? Unless he's just intending on killing them (which of course is a possible motivation if you want to go in that way)... he doesn't necessarily gain anything by doing so. Knocking off their hit points? Okay, then they take a long rest and that problem is solved. Slow them down on their search for the McGuffins? It assumes Strahd knows about them or knows the group is after them, and at that point why not just kill them outright to stop them? Especially since forcing these attacks on them just garner them more XP towards leveling and becoming more powerful. It's hard to toy with someone who just gets potentially stronger every time you do.
But if these attacks of Strahd stop the PCs from becoming more powerful (and they keep getting stuck at a level where zombies and wolves still cause them problems)... the players now have to operate differently while in the Valley. Things like not inviting people into their houses just in case one of them is Strahd in disguise become a thing again. Trying to sneak around and do things without drawing attention to themselves (in order to gain XP in hopes of leveling up before Strahd notices) becomes paramount. And not letting them gain an audience with Madam Eva until they have sufficiently proven themselves by actually reaching a certain character level (and only then do they discover the objects that allow them to try and defeat the Lord and escape the land.)
Having now played 4E and 5E for all these years... I now truly see what Level Drain (and by extension Magic Item saving throws) was meant to accomplish-- hitting players where it truly hurts... how cool they are with all the stuff they have and what they can do. Killing them is too easy. Keeping them mediocre is way worse.
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Now in my first two runs of CoS, I had incorporated the Fanes of Ravenloft (which come from the E
xpedition To Castle Ravenloft adventure for 3.5). This gave the players additional locations to go after and shut down, in order to reduce Strahd's overall power in the valley (for every fane destroyed, Strahd got less powerful). That worked to keep the players motivated to keep exploring and it extended the campaign story beyond just quickly beelining to the Sunsword and the Holy Symbol of Ravenkind and then storming the castle to kill Strahd. But it
didn't solve the problem of the PCs just becoming too powerful for their own good and eliminating most threats in the Valley because they out-leveled and overpowered everything.
But if I can use Strahd to
de-power the PCs when they play poorly and make him a truly frightening foe again because of it (while simultaneously making sure to just not demoralize the players such that they fall out of the game)... it can bring the setting adventure back up to its scariness level that it truly deserves to be.