Success and Failure
Repeated failures or outside problems could lead to the whole goal failing. It happens! But give the player a fair chance. Even if their goal is really hard to achieve—like driving the undead out of Eox—there's a chance they might find a way. Don't undermine their efforts or ideas, but do make clear the magnitude of the task they've chosen. Remember that even if a goal fails, the effort was worthwhile, and the PCs might still achieve smaller successes along the way that open up new goals. For instance, the PCs might not succeed at driving all the undead out of Eox, but in the process, they might discover part of a powerful ritual that might restore the planet's ecosystem and allow living beings to thrive without environmental protections, if it can somehow be reassembled.
A failure or a success at a long-term goal can be a major emotional beat for the character. They've changed the world, after all! Don't shortchange it just because it happened in downtime. In fact, because it might have taken place over multiple sessions, the player might have been looking forward to the results for a really long time!