The necromancer Jekserah has fled Gloomhaven. Before she left, she made some threats against you. It is clear you need to hunt her down before she can make good on her threats. Unfortunately, you have no idea where she is. The city guard isn't much help here, but Argeise does have some suggestions. There is an Aesther enchanter in town who may be able to divine Jekserah's location, but she's notoriously difficult to work with. Alternatively, some of your contacts at the Santuary of the Great Oak know of a temple dedicated to an unseen force that will answer any question asked of it.
You've located Jekserah's sanctum in the Corpsewood. It's time to put a stop to her necromancy for good.
After a grueling fight against her undead lackies, Jekserah falls to your blade. You separated her head from her body and hastily left the wretched place full of mangled corpses. You returned the head to the Captain of the Guard and were paid handsomely for the bounty on her.