Mark on the tree
The marks on the trees can be found in The Shelter of Deep Morkha.
There are 4 in total, with the following text.
The cabin is no longer safe. There is a giant eye that attacks people. I repeat, do not stay in the cabin, it is no longer safe. I'm not crazy, yet.
Watch behind the bushes. There is a giant moving tree. I'm not crazy.
The years I've been staying alive in the cabin. I start counting 4 years after the great invasion. I'm building the cabin, it does not exist yet. My name is Helmut. IIII IIII IIII IIII IIII IIII IIII IIII
Behind the bushes, where the arrow marks, there are many moving trees. I think I'm starting to go crazy.