I do believe you need something like this:
Make a fungus-like creature that acts much like a mc creeper, except like a puffball. It can move slowly, regenerates health quickly, and has a large amount of health to begin with. When they explode, they partially hurt those around them, but implant spores in/on their body. The spores expand quickly and overtake the body, turning them too into a huge fungal biomass. If sounds can be played, make them sound like crying/shrieking babies.