He is pointing out a grammar error in the Japanese names for the oni that are creating havoc in Mooshu
The monster we fight inside Shirataki Temple is called, in-game, "Oni no Plague"
Which is supposed to mean "Demon of Plague" in Japanese, but the way KI wrote it out, it actually says "Plague of Demons".
Same for "Oni no Death" which, as written, actually translates to "Death of Oni".
It's like writing "Loom of the Fruit" when you mean to say "Fruit of the Loom". Or saying "End's Land" when we mean "Land's End". The -no- particle is like a " 's " possessive and follows the noun which "owns" the following noun.
The correct word-order would be "Death no Oni", "Plague no Oni", etc. (Actual entities of elemental horror in Japanese culture have specific, proper names that shall not be repeated here.)
Currently NPCs and treasure cards mention "Oni no War" and "Oni no Death". But as they stand, these terms actually mean "war of the oni" and "death of the oni" respectively, since the Japanese particle "no" 「の」 is more like the English "'s", rather than "of". The correct phrasing would be "War no Oni" and "Death no Oni", respectively, though I do have to question why they mixed English and Japanese words.
Currently NPCs and treasure cards mention "Oni no War" and "Oni no Death". But as they stand, these terms actually mean "war of the oni" and "death of the oni" respectively, since the Japanese particle "no" 「の」 is more like the English "'s", rather than "of". The correct phrasing would be "War no Oni" and "Death no Oni", respectively, though I do have to question why they mixed English and Japanese words.
If I had to guess, KingsIsle used "no" as a direct translation of "of". While this is correct in some cases, most of the time, "no" is a possessive, as you've said. However, they probably mixed English and Japanese words because of Mooshu's severe Japanese influence. Oni is a Japanese word as well: Demon. Demon of War: Oni No War Word for word translation.
Or saying "End's Land" when we mean "Land's End". The 
