Doma is a city in Final Fantasy VI. Final Fantasy VI had many characters, and one of them, a knight named Cyan, comes from the city. The game has many places to visit, and the city of Doma only shows up a few times in the game. Despite the fact that Final Fantasy VI had a general science fiction theme, the technology level in Doma seems to be quite medieval. The entire town actually appears to be nothing but a rather stodgy looking castle.
The plot of Final Fantasy VI, as if often the case, involves a war of some free towns against an Evil Empire. Doma is one of the cities that chooses to fight against the Empire and is destroyed for it. It is this destruction that prompts Cyan to join the party that player controls. Again, this is rather cliche, since "Hero's home village burnt down" is at the head of RPG cliches. What is significant is that Doma is not burnt down, but that the evil general Kefka places poison in the water supply, quickly killing off most of the residents.
I have heard various theories about the reasons why Weapons of Mass Destruction are often present in Japan's popular culture, and while the answer may be obvious, it is an obsession that seemed to have started in manga and anime that took a while to work into video games. The games in the Final Fantasy series up until this point were mostly flights of fantasy. By adding a chemical weapon attack to the game, the game designers were helping up the level of realism, conflict and commentary on modern history to the series, which would help the breakthrough to the complicated parable that was Final Fantasy VII, and other realistic, modernistic RPGs.