The maximum sync speed of the D800 is 1/250th sec. When using a shutter speed faster than 1/250th sec the rear shutter curtain starts closing before the front curtain is fully open. When the front curtain is fully open, the flash fires. Since the rear curtain is already closing and covering part of the sensor the result is the dark band that you see across the image.
Keep in mind that some cheaper RF triggers have a slight delay which will require using a shutter speed between 1 (1/3rd -stop) and 3 (1-stop) slower than the maximu Sync speed. Consult the RF triggers User Manual . Good Luck and Enjoy your Nikons!