Before there was Curse of Strahd (CoS), possibly the most famous 5e adventure, there was the 3rd edition adventure Expedition to Castle Ravenloft. I run this adventure around Halloween, sometimes starting with Death House, the free introduction to Curse of Strahd. It's fun for players who have never experienced Ravenloft before, and it's got a few surprises for players already familiar with Curse of Strahd.
When using the book with players who own Pathfinder rulebooks, the adventure is ready run as is. Very minor conversion is required here and there to account for differences between 3rd edition and Pathfinder. For instance, CMB and CMD don't exist in this book, and Spot and Hide checks are mentioned instead of Perception and Stealth. As long as you've run a few Pathfinder games, you don't even notice the difference as you naturally ask for checks that make sense according to the Pathfinder character sheet and Core Rulebook.
When using the book with players who own D&D 5e rulebooks, the adventure does require conversion, but less than you might fear. Almost invariably, you can just substitute monsters mentioned by name in this book with monsters of the same name from the Monster Manual. A zombie is a zombie, a ghoul is ghoul, a ghast a ghast, a rat swarm is a rat swarm, and so on. If anything, I feel like 5e is safer for player characters than previous editions, so you're most likely to experience underpowered encounters initially. But as you play, you get a feel for player character abilities, and you can either add another monster to an encounter as needed, or else buff a monster with an extra ability or use a similar monster with a little more bite (yeah, I know) to it.
I think Expedition to Castle Ravenloft is a great alternative to CoS, and in some ways I even prefer it. Over the course of four blog posts, I'm going to review the book chapter by chapter. There will be minor spoilers in these posts, so this is primarily intended for Dungeon Masters.
The introduction to the book explains how the adventure's lineage, and specifies that it's intended for four 6th level characters. Strahd is CR 15 in this version of the game, so starting at level 6 ensures that players have some leveling up to do before facing Strahd, but that they're able to withstand, say, hordes of undead (should the need arise) in the meantime.
The first section of chapter 1 provides Strahd's stats and talks about important NPCs who serve as agents of Strahd early in the game before players are equipped to face Strahd in combat.
If you're playing this in 5e, Strahd is the most significant exception to swapping out monsters with an easy equivalent from the Monster Manual. The 5e vampire lacks many of Strahd's abilities (one of which is a key component of this adventure). The spellcasting vampire variant is close to being right, but I feel that even it lacks a little flair. My preference is to use my own 5e stat block for Strahd, which incorporates some of his immunities and adds in a powerful polymorph form not mentioned in CoS. Alternately, you can convert him from 3e yourself.
Strahd's agents in this book include several generic spies (dire wolves, worgs, swarms of bats, ghasts and ghouls, vampire spawn, wraiths and wights) who monitor the players from varying distances throughout the adventure. There are also a few named NPCs allied with Strahd, and they're pretty terrifying.
Varikov the Trapper is a depraved ranger, with his wolf companion Farkash. His favoured enemy is humans, so he gets bonuses to tracking the player characters.
Kavan the Grim is a daywalker who hurles himself, top speed, at the player characters when they least expect it. He's less a spy than an assassin, so he may not fit in exactly with every campaign, depending on what Strahd's motives are (more on that later), but he's a great way to broadcast to your players that the vampires of Barovia aren't the standard Monster Manual fare.
Sasha Ivliskova is the classic Vampirella or Bride of Dracula nemesis. She's outwardly beautiful, and avoids direct combat herself. She travels with two vampire spawn, though, and she'll monitor the player characters as needed.
So why are the player characters in Ravenloft? Well, if you own Van Richten's Guide to Ravenloft for 5e or Horror Realms for Pathfinder, you can easily come up with characters who are natives to the realm. Or the player characters may have been transported to the realm through mists, as in CoS. Either way, Strahd's primary goal is what Strahd's primary goal always is, and if you know Strahd's lore then you know what that is so I won't go into detail here. The book also provides 1d6 possible secondary goals for Strahd, and each goal corresponds to a letter he writes to the PCs to lure them to Barovia.
Here's a problem, though. The letter that summons the PCs to Ravenloft isn't signed by Strahd. Instead, he forges a letter from the burgomaster of Barovia. The weird thing is, Strahd's already killed the burgomaster, and the burgomaster happens to be the father of Ireena Kolyana, who Strahd believes to be the latest incarnation of Tatyana. Why would Strahd summon the PCs in a way that leads them directly to Ireena? It makes no sense in all but one of his 1d6 secondary goals, and it seems awfully risky to the single most important thing in his miserable, doomed life. I can't believe he would risk losing influence over Ireena by leading a group of heroes right to her.
There are 1d4 easy fixes for this:
No problem.
If an alternative start to the adventure threatens to make it harder for the PCs to happen across Ireena, you can of course have any number of NPCs mention her in conversation, or you can just have her show up at an opportune moment.
When you use one of my 1d4 alternate adventure starters, you must still roll 1d6 to determine Strahd's secondary goal. He should still have an opportunity to have a secondary goal, and him having some extra motivation helps you roleplay Strahd and understand his tactical movements. He just shouldn't forge a letter from the dead father of "Tatyana."
Whatever the impetus for the adventure, I like to let the players draw their own fate, in part because I prefer players to bear the burden of having crafted the machinations of their own demise, but also because it provides an early and immediate introduction to Tarokka. Instead of using dice to define Strahd's secondary goal and choose the start of the adventure, I have the players each draw a card from my Fey Deck. Depending on how many players I have, I do whatever is necessary to generate a number 1 through 6 for Strahd's goal, and a number 1 to 4 for the adventure start (take the average of two card values, or give a number for each suit, or whatever), and then proceed.
Ravenloft is a great setting, and Expedition to Castle Ravenloft features many of the same great elements you find in Curse of Strahd and adds in a bunch of others. If the introduction doesn't hook you, the next chapter on Barovia surely will.