Not all battles were so easily resolved. Oto was the last to fall and for good reason. The barrier priestesses were not to be underestimated, so plotting the downfall of the Kingdom that commanded them required much more thought.
For whatever reason, Shinryu had not employed and Barrier Eaters since the Otoan War some years back. The reason was unknown but the most likely conclusion was that he just couldn't.
It was speculated that each abomination had to be created individually. As with any experimental project, there were probably failures. And considering the abominations often resembled other creatures, the supply was likely limited.
It was probably some sort of strain or corruption that was used on corpses or living test subjects. Reports showed that each deployment of abominations had used different types, which reinforced this idea.
