HOW TO First dig a star shaped hole 2 blocks down,Then place on block of TNT in the middle of it also pistons on all four sides of the TNT, Now fill in the hole with dirt or something that can be broke easy,Then get a pressure plate and place it on top of the block with the TNT below it,Now you can place something three block high of the pressure plate so players or mobs can step inside on the pressure plate letting the pistons work and trap them inside while the TNT blows up and they die.

Thanks and have fun with the trap ;)