A very simple to use plugin.

Commands:

/deathpos1 (Lets you set the first position where you will set the death block, similarly to worldedit)
/deathpos2 (Lets you set the second position where you will set the death block, similarly to worldedit)
/setdeathblock {block}

Now people who step on the block you specified in the region you put, will die.
You can put multiple deathblocks.