My group really likes to RP and they have their own fleshed out characters. The characters are all very individualistic and the party even sometimes has inner fights(by fights, I dont mean heavy arguments, but actual damage thrown around). This is all cool and great fun. It promotes different ideas, approaches and its all very friendly n professional out of the game.

Another thing is that the party accidentally released undead apocalypse onto the world. They are to blame for it, though they didn't want to do it.

Thirdly, Im a DM that doesn't mind killing players. I think sometimes dice go bad and for some challenge, sometimes people need to die. It adds great dramatic effect. This, however, means that players need to be able to create new characters that join the party.

Finally, I want to make sure that players know that if they don't stop this evil, the world will be destroyed.

Now I was thinking of getting them next to a fortune teller. The Fortune teller would tell them that many heroes will rise and fall in this war. Many heroes will join their group and some of them will fall. But only their group can stop the black plague. If they fail, everybody fails.

Now the question is - What makes their group so special? Why cant the other groups succeed? If 100% of the initial members of the group die, then why do the new members of the group stand a chance?

TL:DR
How can you kill players in group, but say that only this group can save the world, even if the members of the group change?