i am trying to make a floating island in the sky, i need to be able to shape the perimeter of my terrain. is this possible? can i hide or mask area of terrain so that they are not visible?

If people are going to see underneath your floating island too then I'd recommend the entire island be a mesh rather than trying to use terrain.

I 'believe' there's a shader on the asset store which can have a transparent shader for terrain, not free though I think. Thats if you HAVE to use a terrain object.

But yeah the best looking and most flexible would most probably be to create and import a mesh from some other modelling software. Or if you can force your art style to fit, make it out of lots of uneven cubes in a pattern, could look cool too