Does this make sense?
The play ended in such unexpectAncy that the audience fell silent. X
The play had such an unexpected ending that the audience fell silent. > the ending is totally not what you had expected
The play ended so unexpectedly that the audience fell silent > The play ended suddenly !
Or does it sound any better:
The play ended in such an unexpected ending that..... X
The play ended in such an unexpected fashion that .. ok > better than before , but here it is just about how the play ended that you didnt expect.