Your players are in a small inn when a group of priest (good) are yelling to everybody:"If you want to save your life run outta here!". The adventurer, curious see that an army of skeletons, death knights, ogres, orcs and evils giants are starting to enter in the city. After their observation the small group of priest are starting to go in the melee when skeleton enter in the inn.
Skeleton are arriving from everywhere, the priest are starting the battle with them but after a little time all of them are dead or seriously injured. The players have a chance to exit by the window, now they are obligated to make a saving throw vs. magic. If they failed they are staying if success they can go out of the window with another who failed.
When they are out of the inn it, skeleton are running after them to make sure they go out of the town after 30 minutes the town is burning when they are out of the town they see a small fortress that was not already there yesterday so they were wondering how to enter when the portal open wide to let them enter...
After they enter in the small castle they see a pile of dead warrior weapon, armor, helmet and blood are everywhere. If they lost a weapon they find out in the pile. The most intelligent in the party see a large trace of coagulate blood crossing all over the piece and ending at a large stairway were two humans are there he see that their blood has been sucked by a huge creature because of a teeth live there by the monsters. After that a very big shadow enter in the piece :"A blood sucker worm". After the battle they enter the stairway to the next level.
After 3 hours pass in the dungeon (where you can put monsters to make fun) a Skeleton in armor is looking at them on a pile of bone. "You little worm cross my territory in the 400 years I've pass killing people to make my valorous army your the only one who see mee..." A huge sword (two handed) appears in his hands by magic, the final combat starts.
Here his stats:
Hit points: 46 Magic resistance :90% Special power: disintegration once by three rounds Damage: sword +5 (1d100 / 3d6) special defenses: -2 to enemy attack roll (if good alignment), +2 or better magical weapon to hit.
If the party survives one of the group member can take the sword +5 and the fortress explode so every group member receive 1d6 point of damage. After that they see that nothing have been passed and the town is like when you entered the inn...