You broke our plan, please play along with it anyway, we worked really hard on it.
Meanwhile Maple in Bofuri
"We worked really hard on a plan, Maple broke it(again)"
If it's announced that monsters won't attack these towns, it'll draw suspicion to us... That's why, we'll completely eradicate those towns and erase their very existence off the maps instead, nobody will suspect a thing. Muahahahaha!
Clearing the town will only take one hour out of the ten day event? Then the obvious solution is to clear the rest of the world in the remaining 9 days and 23 hours.
Since the event mentioned players can join monster side, maybe there'll be some lord NPCs who do the same.
My personal expectation is that by the end she'll take over some towns, and leave different RPG setup for those towns for players to enjoy as well.
First would be the town that either barely survive or secretly made a deal with her, they'll be the border of human territory and are under occasional attack by undead to keep up the appearance of persistent threat.
Next would be the completely ruined towns infested with roaming lesser undeads, your standard open dungeon.
Then, probably closer to her forest, would be the cities ruled by her direct subordinate (probably Deas, given he's the OG event monster and thus likely have some history with the country) with a human lord as his subordinate.
If she's really into GMing she can even have some residents make an underground resistance just in case some players manage to sneak this deep into her territory.