Yeah, it's like a spear with a large single sided blade. Where spears are best at stabbing, naginatas are best at chopping and slicing. The second head is cool, but entirely optional.
Here are some good examples, in various styles:
[link][link][link][link]There is a chance that whoever uses it is from the over-powered nation, though this is not always the case. Either way, the use of the weapon would invariably cause the nation to fall. Yes, there is frequently a lot of damage done to nearbies in the process, but mostly because the job of ending empires is really good at stirring up wars.