This mod adds a block that is called death.
This block will turn the surrounding blocks into itself.
Any entity that steps on it should slow down, lose the ability to jump, and have damage delt to it while on it.
Hard Death also starts to appear, which deals more damage.
Use DeathSpread command to stop/start spreading. This is automatically set to false, so you have to activate it.
Use Diamond Infused Copper Block to block spreading in the area it surrounds.

This was made using MCreator