I've read a few threads where people have some misgivings about why the 'tomb robbers, knowledge seeking' PCs suddenly become 'city saviors' when the events at the beginning of Empty Graves takes place.
While I agree that some 'help' is needed from the players to steer their PCs to go along, I do think that a stronger link/reason could be forged.
How about if the PCs somehow misunderstand the Scorched Hand's purpose who really want to keep the mask out of evil hands. While they are fighting the Scorched Hand, they somehow allow for an evil character (necromancer) to get the mask and get away. It's best if the PC somehow see the necromancer get away with the mask (give the guy a 'get-away' scroll, whatever that means to your PCs).
Then when the undead plague takes place in Empty Graves, they have themselves to blame and thus have a better reason to help out and redeem their wrong.
Thoughts? Too cliche?