YOU DARE DISTURB ME?!? RIIIIISE!
The party had entered a haunted wood and dealt with several areas infested with skeletons. At the end of this map, in the large chamber in the bottom left was the source: the insane spirit of a long dead necromancer from the old empire. The party had to fight though a few groups of undead (including a foil in the form of a collapsed passage full of undead rats), draining their resources before getting to the final boss.
This was a bit of a change from most of the encounters we had run so far, as the way the campaign is structured basically forces one encounter per day, which is very difficult to balance properly to give a decent fight. I usually have to use creatures a few levels higher than the party, but I’m sure it gets a bit tiresome to be constantly fighting creatures that can always hit you and do big damage. This time, it was pure level-appropriate monsters all the way through.
As for the necromancer spirit himself, while was a powerful creature in his own right the kicker was when he raised his own skeleton at the start of the battle in the form of a deadly skeletal mage. Throw in a few minions, and we had a memorable fight on our hands.