I don't know if someone already said this, they probably have but...
This part of the arc combined with the rest of the series is almost a carbon copy of Owari no Seraph.
Some examples:
1.) "Don't drink it your body can't handle it"~, sound a hella lot like "don't take the second pill, your body cannot handle it"~. There are plenty of examples of this used in films but this is IDENTICAL. Changing the pill to a drink is not really changing anything.
2.) Jeopardizing the little sister by making her becoming some sort of god-tier monster is something both the series have in common.
3.) That catgirl's character is extremely similar to Kimizuki's. Both have an ill sister; both are trying to find a cure for the sister; both are inconsiderate/thick-headed meaning they do some pretty stupid stuff; both have trust problems.
4.) Yuuichirou is to Ashuramaru as Kazuya is to Kiriha.
5.) This is more extreme because even Owari no Seraph probably took these things from some other series but; both series have talking weapons; in both series, the weapon can take over the user if they are not careful; both series once the weapon takes over the user the user becomes some demon thing.
I am sure there is some stuff I missed but that's just the stuff I remember from the top of my head.
I don't know if Owari no Seraph copied all this stuff from another manga, light novel, or anime because I really only started anime when I was 13 two years ago. But even if Owari no Seraph did copy from some other series that isn't my concern but If I did this on a paper for school, I am sure I would be marked for plagiarism. Seriously, I don't know anything about manga production, but in my opinion, these many hardly altered similarities are just too much.
I personally don't like how they are using the same premise over and over again from another mediocre series but maybe the majority does. I am looking forward for this arc to end. This series has been a bit on the disappointing side, but I have my self to blame for even picking it up.