Piratecat
Sesquipedalian
I think I'd be foolish to discuss any details here.Even so, you know its tricks, the full layout of the lair, and the PCs have all their big guns. So I'm left to wonder what might offer in some level of surprise/complexity.

EDIT: what would you do? Come suggest ways you'd build the encounter! Not for my players, please.
The answer, though, is that I ask myself: how does the monster think? It's been threatened in its lair by someone who escaped once and then brought back allies who attacked it and escaped again. Is it self-confident or smart enough to predict future trouble? And if so, will it delightedly wait for the enemy to show up, flee elsewhere, become the aggressor and take the fight to the enemy, or turtle up to defend its lair? Perhaps a combination of the above? A really good dungeoneering* check might answer that question, or the group may have to return to the sewers to find out for themselves.
They know it's undead, intelligent but stubborn, easily angered but probably lazy (especially if agents have been bringing food to it, instead of it going out to hunt for food), and has an awful sense for interior design.
* No one in the group has a dungeoneering check worth a damn, so I like to say stuff like this. It keeps me amused.
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