How do you get sword of fire in castle crashers

If I know which one you're talking about, (Fire sword with a kind of dragon-head thing on the end) the last boss has it... hint hint.

So yeah, kill the final boss and his sword will land somewhere. The reason you may have missed it is if you didn't notice it (Which you clearly didn't), or if you were playing with someone else, they could've picked it up, and it's one sword, one person.

Make sure you look for the sword as soon as the boss is dead, as you have a short time to pick it up, before you start falling that is.