If the entire royal family somehow died, who becomes the ruler? I mean say the king had one son, then the queen died, the king dies then the son dies. Somehow any brothers or cousins to this king are dead, there is absolutely nothing left of this particular royal family. Who would become the ruler?

As unlikely an event this would be, the royal bloodline I'm sure is well documented outside of established titles. Of course protocols change from one monarchy to another, but there are thousands of people I'm sure ready to step in. Someone in the House of Lords would probably be "elected" to act as a regent for the interim.

In modern times such an event would most likely lead to the removal of the monarchy......

But it is never hard to find someone who is descended from royal bloodline, most have centuries of generations and there will be someone.

Depends..it would probably pass to someone else..there are usually huge huge succession lists with hundreds of people

Depends on what country monarchy you're talking about. Most monarchies hold 0 power nowadays, except for ceremonial/traditional power (which is basically nothing as well). Monarchs haven't held power for centuries in Britain, since they lost the civil war.

Assuming a more pre-modern traditional bloodline based monarchy?

If the entire line of succession somehow died? Worst case scenario?

Well, a war of succession as various military leaders and the few surviving politicians and Nobles fought for power. Because pretty much all of the nobility would be dead.

Chances are, a Military leader would gain power.

It's pretty much impossible for the entire royal family to die, considering extended family are also in the line of succession, with hundreds with some kind of claim on any throne.

There is always someone "in-line", they probably have records stretching for hundreds of people and if not it can be easily worked out.

On the contrary, Her Britannic Majesty (at-least) continues to exercise a great number of constitutional functions; if your only definition of "power" is the ability to act like a despot then your probably right though.

Succession back when monarchies were a going concern was often anything but cut-and-dry, as others have pointed out. As to modern figure-head monarchies, it's largest beside the point.

By sheer happenstance, several months ago I stumbled upon a fascinating YouTube video that those interested in this topic might find as fascinating as I did. Based on recently (?) unearthed evidence of illegitimacy in the line of British succession, it traces the family of the legitimate contenders down through the ages until we reach modern times, concluding with the person who should be Britain's current ruling monarch. It's quite well done. The "Monarch without a Crown" is interviewed, and turns out to be an utterly interesting individual. Here's the video:

http://www.youtube.com/watch?v=7DCasz6oeL4

Watch it all the way to the end. The ending (while credits are rolling) brought a smile to my face.

-Decrepit-

I'll watch it but it doesn't sound plausible, considering that the House of Winsor/Hanover hasn't been in power since the Middle-Ages and their original ascension was decreed by Parliament.

EDIT: Or hell Parliament making William of Orange (of the Netherlands) the Monarch despite his equal lack of legitimacy.

Pretty much, they don't have any control over laws and the going on in Parliament. Sure you could point out that politicians "need" to get permission from the Queen, but really what she says (agrees/disagrees) is irrelevant as they can just step over her opinion - because that's all she holds is an opinion.

That's like saying a General doesn't have authority because he doesn't can't wage war on however he likes or the Prime Minister because he can't authorise a nuclear-strike entirely of his own volition.

People need to stop assuming that a monarch that cannot do as they please has no authority, it's an obviously antiquated view that doesn't reflect how remaining monarchies function. Could the monarch be entirely removed from the system? Of-course, does that mean that see doesn't have legitimate authority? No.

Where does their authority stand, besides a figurehead and tourist attraction?

I don't understand how even being a figurehead doesn't count as authority? That and the ultimate authority of the sovereign is enshrined in the same laws from which everyone derives their authority.

Figurehead with no power. I may have used the wrong word/failed to expand upon it. Since the 18th century the throne has had little to no power, this can't even be argued

Nor is it, however their "power" comes from the established law of the realm which is also from where Parliament draw their's surely? No, Queen cannot reign as a despot (though it could be argued that the position of the monarch prevents a Prime Minister from doing so also) but I don't think that that means she has no power, authority as defined by the law she is (technically) at the top and that's pretty clear cut to me.