Need some help with an encounter....

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ok so I plan on having the players in my group come accross this village that's infected with plague and rumors of undead.
Now the encounter I'm trying to work out happens at night in a graveyard. There's a "thing" (I call it a Necrotic Cyst) that "comes to life" at night. Now the "thing" is the source of the plague or the catalyst. However the person who created it, albeit on accident, is a warlock/undead master wererat.
Now here is how I play out this encounter.

Now I want this to be a fairly difficult encounter for level 3 party (4 characters). Pretty much the Cyst has an aura that drains life and bolsters the undead (give them some sort of bonus like Temp HP?). But I'm looking for a creative way to destroy it other then wack it with a hammer. The Cyst cannot move, the undead will be minion types and dumb as rocks (i.e. skeletons and zombies).

I was thinking they have to slay the creator of it, at the portal. Perhaps they have to have a person during the day when it is inactive willingly sacrifice themselves??? I'm looking for something other then the greet and beat encounter here....I'd say ritual but I don't wanna go down that route either.

Now the party has been carrying a dagger (dead magic item), that reacts to magic portals and rituals, so I was thinking this would also be a good idea to bring that out.

Basically no matter who they fight with the end of the adventure is right there in their faces once they find it. I like the idea that they have to do something to seal it involving the creator, this will put them in direct conflict. She is trying to hide her involvement sense she basically disrupted the trade in the town for the gang, who has now sent a group of humans and another wererat leader to investigate.

Putting them in conflict with her, means she'll try to kill two birds with one stone and turn the enforces onto the PCs and hope that they all wipe each other out. But until the confront her she will offer what help she can, so she can try to cover her tracks and blame it on another NPC (a local priest who really just has bad timing).