There is a quote from Heart of Darkness at the start. There are these kids who performed a ritual and unleashed something. Their presence at the bar caused the fight and opened a wound. The wound is a portal to another world. This is outlined in the book The Translation of Wounds II. When people look at the phone they see pictures of things the kids did and it sparks an interest or attachment in the viewer to this released entity. It comes from a tunnel. It tweaks an interest and suggests something about oneself as per the quote from Heart of Darkness. He's a shell of a man filling the void with drinking. The cockroaches are the amount of entity present. Hence at the end they're everywhere and the thing comes out of the wound portal to enter him and fill his void.

At the end I found myself wondering if I'd blinked and missed a vital plot point because nothing was explained (and not in a "ooh ambiguous and intelligent" way).

Good performances from all involved but as a massive fan of Under The Shadow, I found myself very disappointed.

I understand how and why many are not into this. I was also quite frustrated watching this by how certain characters behaved. You really have to not just suspend disbelief, but like throw it away as far as you can. Maybe shoot it out of earths reach. Having said all that, what the movie does succeed in, is to create a mood. Which of course only works on you, if you allow yourself to be taken to that special place.

Not an easy task and most definitely not an easy ask to the viewer. The actors do their best to say the least to convey their behavior at first and then the over the top things that occur because of that. Many questions are raised, not many are actually answered. Well not obviously answered - as in they are left up to the viewer to decide or make of it what they think ... which is either a good thing in your book or something you don't like at all. There is no right or wrong here. Same goes for the movie, that really looks good and builds a nice mood ... but is it enough? And does the ending satisfy you? Is it supposed to?