Hamlet weird shadow line near Herald island

[Mantas 688]

this line just appears there for some reason

[Hyxaru 17]

It’s another island.

it is where the developers store all the interiors of buildings. What you see is an island compromised of ‘fake ground’ turf, and the black line is the edge of the island. The cliffs you could say. Using something like a lazy explorer, you can get onto the island. 

You can remove this invisible turf with a pitchfork, revealing barren turf. After doing that and relogging, you will get a normal island border.