Rectangular shape, opened up at edges to an irregular shape (like opening the edges of a swiss army knife), changed shape again and looked like a fighter plane, then back to retangular shape. It was black, no lights, flying below cloud cover.

Travelling at a similar speed to an airliner, but rotating when shape changed.

Because of the irregular shape, I thought perhaps it was a helicopter at first carrying cargo (I have seen this before) but I couldn't hear any noise. As it came closer I noticed the shape changing and rotation of the craft. It was travelling at the speed of an airliner in a straight line, but the shape changing or rotation is not consistent with any aircraft I have seen before. If it was a spoof/prank I'd be interest to know how they did it.