One card from this player's hand must be discarded to purchase the service of the mercenaries. When played, the player can choose any one type of support card that can be played by them.
Copyrights:
TSR
Wizards of the Coast
Not many Spellfire cards have a cost, so ones that do need to be examine carefully. This one forces you to discard any card (your choice) from your hand. That means that this card needs to be worth two cards to play it for you.
The +9 is great and significant, and the ability to use any one support type you didn't already have access to is nice, but most decks won't need this. Unless you are going with a complete mongrel theme (only the most powerful cards of all types), this one shouldn't be needed.
The best use for this one is actually in Sealed deck competitions (ironic, ain't it?), as you often need to use cards you can't always count on using, and often have something else you can discard.
Other than those types of situations, I don't think a good, focused theme deck should ever need this card.