My first idea would be Alien Parasites that, in term, are basically entities in your normal Minecraft world that are found in Hosts, which are mobs that are infected. You can tell if they are, Blue, Green or Black. There's three types, each of them are useful and can be devastating. First one is a Converter. ( Green ) If you get too close to a Infected mob with the Converter or a Converter parasite, they will latch onto you for a while, and you can simply punch them off. If you let them be, they infect you momentarily, bumps on your skin. Soon you will turn into a Cocoon and become one of them, and either you accept it or you die. The same goes with other mobs, but you can infect mobs as a parasite and even control them. There are some abilities to being a Converter that anyone can add on. Second one is a Mob Converter, ( Blue ) which will turn Mobs into other mobs by cocooning them. Hostile mobs and Passive. can become parasites, or slimes, but you can also make them turn into other things if you want by having a Staff that allows you to choose what the Mob turns into. Third one is Hive-mind parasites ( Black ) that spread a type of converting spore they carry, that changes mobs and things around them. They are very dangerous but also very useful if you use them the right way. Thats all for now!