They find the worlds Keyhole, once they do they can get to the world's heart and consume it. After that the Darkness pulls the physical world itself apart and draws it into the Realm of Darkness.
Heartless are physical manifestations of the Darkness in people's hearts.
edited 17th May '12 7:21:14 AM by Jamdat369
"All worlds begin in darkness and all so end. The heart is no different."-AnsemIf it were only that simple. Only people who can Keyblades can lock Keyholes. Fortunately we have one such weilder. But he can't get to all the worlds before the Heartless.
-toCloud-Yeah you even have the same name. He has blonde hair though.
"All worlds begin in darkness and all so end. The heart is no different."-AnsemWell, I guess the next best option is to kill every heartless. Of course, I suspect it's one of those As Long as There Is Evil sort of things...

-cleans up the beer cans with his Broom Scythe-
Goodness sake, man. All this will do you no good, remember that you can still die of old age, you don't want your liver going out first. You still have a liver, right?
mario is red, i am green, i try my best, but everyone's mean