Despair

If they had been human, they would have realized long ago that something was wrong.

Setting aside the fact that mustard gas could take up to 24 hours to take effect, and thus showed no immediate results... common sense alone would tell anyone to evacuate if the area around them was covered in yellow liquid.

The color of mustard gas clearly didn't look hygienic. Even a human who knew nothing about mustard gas wouldn't stay long in an area where it had been deployed.

But monsters were different.

Monsters had no concept of hygiene. That was due to their nature.

Animals had a weaker sense of hygiene than humans. The same was true of monsters, so it never even occurred to them that they needed to leave the area affected by mustard gas.

And the consequences began to appear in the most vulnerable individuals first.

The first symptoms were severe itching and skin irritation. Most of the monsters had no idea what was happening, even as the young ones scratched themselves raw.

The yellow cloud had also been swept away by the wind overnight. So what could possibly have happened? Since it wasn't unusual for monsters to scratch themselves, none of them paid much attention.

They realized something was wrong when yellow-fluid-filled blisters began appearing on the young monsters' bodies...