I have found out how to make a dangerous end crystal trap.

I would build this in the ground if you want it to be a trap.

First, get about one hundred end crystals in one block, hard but possible.
Second, place a dispenser above them facing down.
Third, place an arrow in the dispenser.
Fourth, place a block atop the dispenser. (this step is optional)
Fifth, place a pressure plate on top of it all.
And now get someone to activate it!

If built correctly, it should instantly kill almost anything, and if the explosion doesn't kill them, it should launch them thousands of blocks into the air!