You may marry him, murder him, or do anything you like to him." - Doyle about Holmes
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Sherlock carefully entered the room to which the clues sent by Moriarty had sent him. He knew Moriarty had probably planned a trap of some sort. When nothing happened for some minutes, he began to relax. The room was quite bare, making any trap difficult to hide - there was no furniture, only four walls, a ceiling, and a window. There was not even any artificial source of light, only the natural light that came through the window. The room was as clean as it was bare, without dust or streaks on the window, and utterly silent save for the noise of his own footsteps and breathing. He paced the place, but found nothing.
His eye was drawn to a prism resting on the windowsill; it was the wrong angle to cause a rainbow, and Sherlock thought that was quite odd, given Moriarty's attention to detail. Then he thought that it was the bait, so to speak, and that "correcting" it would in some way cause the trap to spring. He backed away from the window, lest sheer proximity spring the trap through his vibrations, shadow, or the like.
This was just as Moriarty had planned.
A clear, almost impossibly thin thread snapped against Sherlock's ankle, and before he could react to the noise the wall opened, and a blade on a pendulum was released, swinging horizontally and chopping off his head.
A part of the windowsill under the prism was triggered to adjust by the swing of the pendulum, and the last thing Sherlock saw before he died was a rainbow.