Hmm, not a lot of detail, so i'm going to make some assumptions.

First is that the party has fallen into the cavern. It's not sealed, it's just deep and the walls are unclimbable. There's a hominid skeleton and a This leads to the second assumption that the party is low level or this would not be a trap for them. And the third is that the journal belonged to a previous victim who died there.

So. The ink should be made from the blood of some common creepy crawlies that live on the big pile of bat droppings. The party will find the remains of some of them ground up by the journal writer.

The journal describes the writer's increasingly desperate attempts to escape along with indications that they were going insane. Eventually they declared themselves supreme ruler of the bats.

The last entry explains how the writer has been able to encode a cantrip into the journal, one which, with the aid of their loyal subjects' offerings, will enable their