[X] Investigate the migrating earth spirits. Together you can try to tease out the source of their agitation, and save the cartographers some work.
I believe this to be the primary necessity, of the latter two, I support culling the worms before dealing with the undead. Theoretically, the worms could be a problem for the forces in the field, whereas the undead, while not a good thing, are occupying a ruin that we have not been informed of any plans to hole up in. Having the surrounding terrain clear means that, if we don't get to the fort, the main force doesn't have to worry about the worms as a complication.