How do you want to die?

I would guess all they would have to do is get under you. Then use the force of gravity against you, and the air pressure as well. The only problem would possibly be the air pressure on the side of the blade may make it difficult to hold the blade steady, and possibly cause a microscopic turbulence causing the blade to waiver if it came close to your neck. Never having swung a sword upward in terminal velocity, I'm not sure how difficult it is, either. As mentioned before, they are ninjas, and do possess the infinite power of Chi, so even with the turbulence and wind resistance, they may well be able to make contact with your neck. An easier way I think would be for them to have a small patended ninjachute(nin-ja-chute), in which case they would stealthily(this being easy as you can hear next to nothing when falling at terminal velocity), come up from above you. By narrowing their bodies, their terminal velocity will increase, enabling them to compensate for the distance between themselves and you(as seen in the movies). Then they would quickly draw their ninja laser sharpened blade to your neck as if about to perform a slicing motion. At that time they would use Chi to stiffen all of their body except for their non-sword hand. Their non-sword hand would then release the patended ninjachute, and their velocity would drastically decrease, creating a swift acceleration of your neck to their blade. The force(through F=MA) should be enough to decapitate any person with a neck up to 1 foot in diameter. Of course having not crunched the numbers, I cannot give the exact force, and having not experimented I cannot give the actual results. However, I hypothesize that there is atleast a 99.47% chance that the blade will decapitate you.

My personal death... I've always wondered how it would feel to drown. I can't really imagine how it would feel to not be able to breathe. I know you would black out, but I wonder how that would feel, and what you would think given the fact that you would have time to think about your death.