This is a personal, largely spoiler free (or spoiler-vague) review of the Souls of Darkness fanmade campaign by JackofHearts for Arkham Horror the Card Game.
In Souls of Darkness, the investigators begin by snooping around an auction full of relics - only to get involved with a plot of vampiric blood mages that takes them all the way to Romania to fight against Uvhash. The campaign features a wide variety of mechanics and themes which provide a great variety, but at times feel overcomplicated and scattered.
The Souls of Darkness campaign is split over three types of scenarios - it begins with running around Arkham investigating a blood mage conspiracy, with scenarios that have a strong theming and immediately present a good hook, feeling like a natural addition to the Arkham lore. The campaign transitions to Romania, where the setting changes from Massachusetts streets to small villages and intimidating castles. In between these two, and capping off the finale are scenarios with trippy astral travel that feel slightly out of place - meant to serve as a bridge, but not quite fitting in with the rest.
The primary new mechanic of the campaign is Siphon, and it's one of the campaign's strongest points. Siphon is a keyword that activates whenever your investigator or an ally is assigned damage - enemies might heal, get doom, or become stronger after it triggers, which helps emphasize all the blood magic of the campaign. It's not just enemies, but with treacheries also playing into it too, and some scenario specific surprises throughout. The Siphon design, and how it's used is the absolute standout here - while not every scenario introduced a new twist on it, it felt fresh throughout the campaign, and was my favorite part. The campaign also introduces Power - this acts as an alternate victory-like track, representing how much occult power your investigators accumulated through the campaign. There's some places to pick it up, and a few scenario cards that care about it, as well as some story paths that might open or be locked out based on it - but overall, it didn't feel as crucial, like its potential was unrealized.
SOuls of Darkness's biggest strength is its scenario variety - each scenario is trying to do something new, and where there are similarities, they're playing off of and building on each other. Avoiding the suspicious Arkham police is similar, but not quite the same as avoiding the suspicious servants in a vampire castle - and while some of your strategies and tech cards will work on both, each also presents different challenges and threats. There's a wide range of new mechanical ideas here, from a scenario that borrows mechanics from Clue, to one with a day-night cycle that features two encounter decks. In addition, the ramping up of the threat of Siphon is great - it turns out that damage is something that basically every investigator can choose to tech against as the scenarios progress - so the campaign's main mechanic can easily be interacted with as you gain XP.
Unfortunately, this wide variety of scenarios also tends to be the campaign's downfall, as some of them feel a little underbaked. There's some frustration in some where enemies who are meant to patrol can move erratically in a way that make them difficult to sensibly interact with, and while the Clue inspired scenario has a neat premise, there are mechanics that undercut the investigation game that's set up, and even force a heavy risk on you if you don't. Many scenarios also suffer from just having too much going on - a large number of cards and new mechanics, many scenarios feel overdesigned - like they're stuffed with one idea too many, not giving them enough room to breathe. The campaign just feels like it throws too much at once sometimes - there are multiple "boss encounters" that have an overwhelming number of enemies at once, or that have their mechanics cross in a way that end up contradicting the design.
Of the scenarios, it's definitely the opening ones that are the strongest - the campaign puts its best foot forward, and even with the roughness of some of the early mechanics, they're just absolutely dripping with flavor. The weakest scenario is the finale - it's got an extremely interesting premise that I liked, but it just didn't feel as if it fit in with the rest of the campaign as a whole. Rather than focusing on the campaign it was meant to be capping off, it brought in other Arkham lore that hadn't been mentioned at all up to this point - drawing attention away from what the campaign did so well, and instead of highlighting its excellence, instead presenting something new that wasn't bad, but was a bit incongruous - which ended up being a good representation of the campaign as a whole.
Overall, Souls of Darkness is a good campaign to give a try if you want to play something that's trying to be experimental with mechanics, and don't mind if a scenario might feel too busy.