Normal: "The leader of the Mi'ihen Highroad fiend colony. It uses its workers to expand the tunnels of the colony. Its fire attacks are a dead giveaway that it's in charge, foo'." Oversoul: "The leader of the Mi'ihen Highroad fiend colony. It uses its workers to expand the tunnels of the colony. Fire magic is its strong suit, but push it and it uses more formidable magic." — Scan
The battle starts with an ambush. Two pieces of equipment are recommended—something to guard against Sleep, and something which absorbs Fire damage. With these in place, the battle will be easy.
If the Vermin is Oversouled it will gain Triple Attack, Holy and Flare. Triple Attack is a physical attack, and Holy and Flare can be blocked with Reflect, although this will also bounce back the Vermin's Fire spells, which heal it.
In the Fiend Tale endings, a notable King VERMIN! was an assassin in the fiend world who had a perfect track record in taking out other fiends. However, he ultimately met his end at the sting of a Death Dauber whose father was his first hit.
King VERMIN! takes its name from a famous quote in Final Fantasy VII; when Cloud's party are caught in a trap set by President Shinra when trying to escape the Sector 5 Reactor, Barret calls the president "King VERMIN!" Likewise, the use of "foo'" in the description is a reference to Barret, whose voice and mannerisms in English-language Final Fantasy VII media are infamously likened to those of Mr. T.
Strangely, the Final Fantasy X-2 Ultimania Omega "Fiend Encyclopedia" refers to the enemy by the name "Magma Mantis". This is neither its name in English or Japanese.