The Fire Kings are more of an excuse for violence than a true guild. Their favorite activities include arson and… more arson. They are led by Krieger the swordsage.
They offered 500 gold to the party if they would betray Giovanni when they were on their way to deliver a message to Badurth at the the Festhall of Falling Coins but were turned down.
They were one of the gangs who teamed up with The Brotherhood Janko to defeat The Gray Orb. After the fight with the Gray Orb, Giovanni double crossed the Fire Kings (and The Kindred of Yoj), and was nearly assassinated as a result. Later, the two gangs beseiged the Brotherhood at the Gray Orb manor. The Party helped Giovanni and the Brotherhood Janko to defeat them in exchange for a cut of his profits.