The whole subterranean village and the connected caverns are a massive Eldritch Location. Beyond the simple fact there is no known long-term human habitation of Svalbard until the 17th century at the earliest, the use of impossibly-huge narwhal tusks and the Unnaturally Blue Lighting of the glacier above show just how wrong it all is.
The boxes from the "amphitheater" deserve special mention. Already strange for how geometrically-perfect they are, despite the lack of tools required to create such things, the fact they contain a human heart and teeth is a sign that whatever was happening down there was bad. Oh, and there are hundreds of boxes in the floor.
The strange noises that the bunker receiver catches while Walter and Jónas are holding down the fort and awaiting a response. Whatever is sending these...mocking messages, It Can Think well enough to taunt the Fristed repair team.
Karina sees a hallucination of someone in the bunker, back turned to her. She assumed it was Casner, but after being told that Casner is in a different room, she goes back to find no one there. Based on the body language of the figure, she was likely seeing the monster...
The moving statues that they discover on one of their trips to the subterranean village. They just happen to turn towards the entrance and instantly everyone decides to leave, because there is no possible way those large, heavy statues could just appear there unless they had moved themselves somehow. Later on the statues, combined with a cave-in, block their escape the last time they come down to try and rescue Walter.
Karina's abduction. She's taken in the middle of the night, screaming for help, and they only thing they ever manage to find of her is her heart in a box. It's still beating.
Walter has a similar fate after he's taken, though unlike Karina we get to hear a snippet of his heart being torn out from his camcorder.
The night before the team prepares to make the trek away from the outpost on foot, they are awoken to the sound of knocking on the door. When they look to see what's out there, they see Karina in her nightclothes, repeatedly asking in a Creepy Monotone to be let in. What's worse is that everyone hears her speech in their own native language, while the actual audio recording is significantly more garbled.
The monster's ability to do basic imitations of people is this in general. Rosa is captured after spending some time having a conversation with what she thinks is Jónas, and the sound it makes when it reveals itself is bone-chilling.
Jónas' infection in his hand from one of the creepy statues, which he initially thinks it's just some kind of frostbite, eventually starts to spread all over his body. When crawling through a particularly tight tunnel we're treated to the sound of his skin sloughing off, and in his final notes he records how what's underneath is identical to the monster.
