"living under a rock" it happen like 15years ago or most. To resume it, since apparently there some similarity between the two story, like some chara design or the fact both story have demons inside human bodies, the fan of naruto said that Seishi have plagia his brother Masashi. Even when they saw a black flame who burn everything in 666 satan, they get mad and said he even plagia the amaterasu from itachi but they forgot to look at the date of release where you can see that the "amaterasu" appeared first in 666 satan.
The manga was good and could be a huge thing in that time if these peoples didn't show up and just shut up, could have gotten another long anime like bleach, naruto, one piece, hunter x hunter, reborn, d gray man, etc... from that time.