A roundup of who can take Defensive Stance/what they get out of it:
Marion Tavares (not on Arkhamdb yet):
Unsurprisingly, it looks like it works best as an emergency "I really need to pass this foot test" for combat-centric investigators, though it might bump the occasional Locked Door or Overgrowth into a success too.
I haven't seen this mentioned, but this card would work really well with Agnes Baker. Cancel out a treachery AND trigger her ability at the same time? Great combination.
This is basically a triple Ward of Protection, but costs 1 resource and 3 health more. Obviously you can have only 2 Ward of Protection in your deck, but together with Purifying Corruption you can cancel up to 5 treacheries! This is pretty much 1/3rd of all the treacheries you will draw. Adding Agnes Baker ability, you additionally deal 5 damage while doing that. Amazing stuff.
Assuming you don't recharge the card and only cancel out treacheries three times, the full price for Purifying Corruption would be 4 resources, 3 sanity and 3 damage. The health cost especially might be a bit high for our brave waitress, but you also have access to Heirloom of Hyperborea and assuming you have more soak (which you should, if you're playing Agnes), this is definitely something she can handle.
This card also pairs extremely well with two particular allies - Peter Sylvestre and Jessica Hyde, both of whom Agnes Baker wants in her deck anyway.
The only real drawback of this card would be the fairly expensive resource cost (4), which might be a bit high for her at times, as she generally tends to be short on supplies, especially if you run her with Fire Axe or Dark Horse, which some people do.
TLDR: So overall 4 exp and 4 resource cost for a card that will help trigger Agnes Baker ability and cancel out treacheries? Definitely something to consider.
Also, if by any chance you are playing with Carolyn Fern on the team (which is a quite common duo), all the drawbacks of Purifying Corruption magically disappear.
Q: Does this item's effect combine with Masks? And if so, can I use Wolf Mask on a test?
My understanding is the mask's triggered ability resolves "for this skill test" so I would be getting +1 to all skills during the skill test.
On the 2nd question - I know there is a rule that forbids commiting cards without proper icons to tests but I don't recall any restriction on using assest to boost a skill irrelevant to the test.
I am absolutely loving this card in my Parallel Roland deck. Both versions of Roland's deckbuilding can take it, and Red Tape makes it a fast play (so it only costs one action), which makes it just about the largest quantity draw in the game... and one action, two resources, and one card in return for five cards is also just profitable.
I slept on this card so hard because I fixated on the idea that I was giving up a card and a resource just to (what I felt like) was just trading in one encounter card (which would probably be fine anyway, since Guardians tend to be pretty mythos resistant) with another, but I've come to really appreciate how much this card does.
It does cost a card and a resource, and you don't even know if the card you were going to get anyway might have been an enemy or a treachery that would have been easily solvable. And yet, this is so, so worth it.
Firstly, in a vast majority of scenarios, the odds of whiffing is absolutely miniscule. The math would depend on the exact configuration for each scenario, but even if you have an encounter deck with 40 cards with only 4 enemies in it, you have an 89% chance of hitting! I'm not sure of the exact numbers, but I think a vast majority of scenarios are going to have better odds than that.
In my opinion, what makes the card worth it, is that the enemy spawns engaged with you instead of anywhere else. That means that if it is aloof, you're not wasting engage actions. If it spawns away from you, in an empty location, etc., you're not having to go off-course, probably wasting multiple actions, just to chase it down. If it has concealed, it doesn't resolve its concealed keyword and make you hunt the shadows. If it is added to your hand, it bypasses that! (iykyk)
Lastly, a vast majority of guardians/killers are going to get some form of reward for killing/fighting enemies. Roland can use this at a high-shroud location to get a clue, it can be used to pop off with Scene of the Crime, Evidence, Glory, etc., Zoey gets her money back, Tony gets the free action and gets the resource back with his Bounty and gets to trigger Lucky Cigarette Case for the card, plus any other "succeeds by" bonuses he might have (and he's actually otherwise soft to the actual treacheries!)
I feel like this, along with Kicking the Hornets Nest (and the new card draw one to come in the Drowned City), require actual play with cards that synergize with them to fully appreciate, as you can't just run the numbers of "is this amount of damage worth this amount of actions/resources" like weapons, clue-finding events, etc.