Rippers - Rippers are small snake like creatures ( No bigger than a foot in length ) that barrow in the ground. They have razor sharp teeth and powerful jaws that can easily crush rock. The arrive here in meteors and are the base of the oni invasion because the chain of power starts with them. Now when a ripper is killed, it acts like a star fish and grows back, however, it multiples. If it is cut in half, it does not simply grow back in two different rippers. Many small appendages grows from it's body that then becomes other rippers.
Next thing a ripper does is mutate. It's mouth is filled with a weird acid. This acid is very weak on the PH level, but it does sting slightly. However, this is used more for mutation as it comes in contact with organic material, it mutates them, causing different types of oni to be created and brought about to life and thus the invasion continues.
Needless to say, the need food and therefor poop. Their waste is highly radio-active material. This material, if exposed too for great amounts of time, will mutate organic material as well. Thus the rippers use it to fashion together the walls of it's hive.
And if that wasn't enough, the rippers bread and create large beings by using the DNA from the organic material that they ate and putting it into sex cells in which they combine together and form new beings