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i have heard/ read that it was something in the past that affected them.

Can you please elaborate a bit on that? Your question is rather vague and confusing.

According to Za Warudo R:2's official backstory, monsters began attacking as a result of the Shadow, which was created by the death of the Gods, as without them there is no light in the world. However, this is all set after R:1's timeline, so it's not relevant to Sign. In R:1, the monsters are just there. There's no given reason for their existence.

Actually, that part of the story is apparently a retcon for R:1. Before R:1, the gods cursed the elves to become humans and then cut themselves off from the earth, causing the Shadow to form. A young Wavemaster summoned new gods, seen in The World, and then after R:1 they vanquish the shadow and go to war with the gods and then each other. About a hundred years after R:1 takes place, R:2 happens. Humans are at war before, during, and after all this, though, so unless they have stupid gate-based wars I assume there's a way to pass the barrier.

Okay, I can jive with that. The backstory gave me the impression that all the crap with the Shadow and monsters was inbetween the two versions. But I haven't actually read it in almost two years.

If you read ai buster which takes place before sign, monsters and humans seem to be at war.the chaos gates were made to hold them at bay with a magic field to protect the towns. once a month the monsters would break the barrier and invade the town, the only way to save the town would be to beat the leader which was usually near the level cap. so they could still be at war or whatever, due to the shadow which has already been explained above