The most disturbing and obvious lead we have are the symbols found on both Sam's head after what appears to be an assault at the Wroth-Caleb crypt, and on Allison's when she was found. They appear on the forehead, drawn on with a red pigment. Thanks to Sam's camera running after she dropped it, while the look at her assailant wasn't clear, we could see both the mixing of this pigment (though with unknown ingredients) and the actual drawing. This is sort of a godsend, given that the symbols became very smudgy, and catching them in the act of being drawn helped clarify the first two that appear on Sam's head.
Screenshots of these symbols from 'Session Nine: Wroth-Caleb':
And these, same video, from when they were being made:
Screenshot from 'the tree', the symbols on Allison:
The rightmost symbol on each appear to be the same, and this is likely for the middle one as well, though too blurry to tell for certain with that one. The one on the left is different.
When I first saw them, they were immediately familiar to me - they very strongly resemble Runes. I won't conclusively say that they are, it could be a freaky coincidence. For anyone who doesn't know what those are, the Runes are the ancient Nordic/Germanic mode of writing. Readers of J.R.R. Tolkien's work would recognize them as a decorative motif. They're still used today in modern eclectic neopaganism, divination and Asatru, amongst other things. They stand for pronounceable letters as well as having symbolic meaning.
The runes of the Elder Futhark (that is, the oldest known set of runes - a set of letters is an alphabet because it begins with alpha and beta, a set of runes is a futhark because it begins with F U Th A R K) that these resemble follow.
The symbol definitely in common resembles Haglaz, the letter 'H'.
The middle symbol, very clearly on Sam and vaguely on Allison, resembles Laguz, the letter 'L'.
The left symbol on Allison resembles Naudiz, the letter 'N'.
The left symbol on Sam resembles Ansuz, the letter 'A'.
So, on the rare chance that the unknown assailant is using them as a simple cypher, we have acronyms ALH and NLH. Now what that might mean, your guess is as good as mine.
However, I don't think that's what's being implied here. Note, I'm currently running off of easily googleable sources, I don't have any books handy to hit (I might need to schedule a library trip), so what I'm running off of is wikipedia, top search results, and misty memories of back in high school when I had pretensions of witchcraft (hey, it's a phase some kids go through) and briefly used them to divine with. So take my assessment with a grain of salt until I've got more to back things up with.
But. What sends off alarm bells in my head that their use here is symbolic, possibly even meant for some sort of ritual or magic, is the first rune on Allison's head, Naudiz. When I used the rune for divination, that was the one you didn't want to cast (like coming up with the 10 of swords in tarot). It refers to need and distress. And it was found on Allison's corpse, and the only variation between her head and Sam's.
The other runes' 'on the surface' symbolic meanings are: ansuz = related to the Aesir (the gods), Haglaz = hail, Laguz = water. The A meaning makes sense in context if we assume that either something supernatural is at work or the assailant was attempting to work some kind of magic. The L meaning could be related to the tragic circumstances of Allison's initial disappearance - being swallowed by the waters near the Blue Hole without being seen going in, water too shallow for her to disappear under. I'm a bit lost about the H meaning - it refers to literal hail, the icy stuff from the sky, so I'm not sure how that fits in yet.
I will have to delve further the next time I have some free time - as a homemaker I'm not blessed with as much of it as you'd think. I'd like to find sources about the runes that aren't fluffy neopagan new age repackaging, or purely academic and from a linguistic standpoint. I'll look for a reliable translation of the old rune poems - the works in which the ancient peoples who used them recorded their meanings. I should also cross reference the other Futharks just to be safe - the Elder Futhark is the oldest and the one most commonly used by modern magic practitioners, but there's also the Younger Futhark and the Anglo-Saxon Futhark to consider. And it's possible that this is a coincidence and they're not meant to be runes, but I find the resemblance too striking to ignore.
areudirty
snowghoul
yousharknotpass
gamechangershow
stormborns
vaporwave