Note: This event card appeared roughly one month after "An Amusement for the Nobles".
You'll have roughly a month between the potential start of "An Amusement for the Nobles" and the next major event card, "Monster Invasion". Do what you can in that time, whether it be claiming new territory, appeasing your leaders, or catching up on quests and exploration. When the "Monster Invasion" event card appears, you'll need to return to the capital to resolve it, and like with the "Troll Trouble" event card, it'll eventually be accompanied by other, repeating event cards that serve to sap your kingdom stats until the issue is resolved.
Don't leave it unattended for too long, and when you return to your throne room after this event card has appeared, your envoy will brief you on the matter. He pulls no punches, the plague is spreading and people are fleeing your lands. The very future of the barony is at stake. On the plus side (and this is a dubious "plus side" if ever there was one), goblins have started appearing in the wake of the monsters, cheering the return of one "Lamashtu".
While the goblins are likely just cheerleaders for Lamashtu rather than active collaborators in the current catastrophe, there's still a chance they know something useful so at least you have a lead. This starts the quest Mother of Monsters.
Immediately after your envoy departs, Jhod will arrive with more, related bad news. He'll give you the details of the disease's progression and reveal that him and Tristian have established a field hospital in an old prison, presumably so they could study the disease and treat patients or more accurately, perhaps, so they could isolate victims. Fortunately (another dubious upshot), a patient has volunteered for vivisection, which is a sign of just how desperate the situation has become. Whatever the outcome (and for whatever reason), Jhod wants you to be present during the operation. This starts the quest The Seed of Sorrow, which you have no choice but to complete.
Before you go watch one of your citizens get cut up by a priest of Erastil, Jhod will also give you a letter from Kesten Garess (Kesten's Letter), which starts the quest Witch Hunt. Seems there's no shortage of issues vying for your attention. After this letter is handed over, you'll finally be able to question Jhod in-depth. The most meaningful of these dialogue option (aside from a [Neutral Evil] moral choice to suggest making vivisection somewhat less elective) is to suggest calling on the services of Bartholomew, should you have convinced him to stay in your lands.