deaton drawing a line of mountain ash around the cernunnos pendent meant to represent their plan to both save and stop the kanima so fun. like he's talking about how the fact that the kanima can't swim may be a glimpse of this supernatural world that's bound and balanced by these rules, and yet the mountain ash is used in the same episode to create these almost narrative opposites. if the water was this protective barrier against the kanima despite jackson being captain of the swim team, this may be a glimpse of its master, so maybe a physical barrier for the kanima could help them catch a glimpse of it again. learning later its master is matt, we know the master in question -- who they're really trying to stop with this plan, now knowing a kanima can even need saving -- can easily cross mountain ash himself, it's just the kanima that can't, which is who they're trying to save. deaton doesn't appear to know much about kanimas despite being a druid; he asks other characters repeatedly what the argent's bestiary says about it more than he offers any knowledge himself, but he asks all the right questions. prod at the one weakness an unkillable creature that won't stop killing has exhibited, and yet he doesn't follow that trajectory at all and instead looks in the opposite direction for insight.
the supernatural world in general is held to this mountain ash rule despite the different kinds of creatures introduced throughout the show, and before this, all they've known for certain is that the kanima is a shapeshifter like they are. but deaton's a druid, and he's a retired emissary, and he knows how mountain ash works. it's an ingredient that's also a spell. their plan to catch instead of kill the kanima in raving is investigated first by this chink in its armor through water, and instead of plan a way to utilize it, deaton observes this unseen supernatural rule and flips it upside-down in order to understand it further. as above, so below, etc... if they can encounter this sliver of its master through the kanima, maybe they can encounter the master through this sliver of the kanima like a mirror held up at an angle to see around a corner. and they do for the first time in the season! even as the mountain ash part of it specifically gets sabotaged and the kanima escapes, it's the thing that allows them to isolate it just long enough for matt to confess a motive to them directly through jackson's body; he's killing the people who 'killed' him.
before the bestiary, the kanima had been thought of as a thing that kills just to kill, and in a way it is, but this is only true because of matt, its regular high schooler not killing anyone friend. this motive is the only reason they're able to separate jackson from the person actually doing the harm and save him, that scott and stiles and his dad are able to put the right pieces together in fury to finally make a case against matt, guy who drowned surrounded by beacon hills' 2006 swim team, despite the actual crimes being exclusively committed through jackson. water as this looking glass through the kanima to its master, to mountain ash used for the same, to seeing jackson pull matt out of the pool at the party and notice it's significance at all. it's enough to ask a sheriff to look for evidence of the one victim matt actually killed with his own two hands in the same episode the kanima is finally freed from matt, who's killed for a second time, this time on purpose. because scott decided to ask his veterinarian boss to help them save jackson instead of kill him, and because deaton let himself be an emissary again for this high school kid who decided it was worth it to ask. because deaton did enough detective work in the supernatural world that a detective in the human one was able to parse the two apart and see the human committing non-supernatural crimes. deaton took one look at the kanima's one known weakness next to not recognizing its own reflection and was like what if this plot device too was a mirror motif.... literally yay television <3 teen wolf season 2 and specifically episode 8 raving you will always be famous
