Here's what you do for halloween...
Get in the players heads.
Let them find that skull, but you dig into your notes from the past 200 games for the story. Everything is familiar and nothing is quite right. Things get wierder and wierder until they find themselves plopped in the middle of the iron siege, unprepared, except its in the middle of Dal Quor. Also archons, devils and demons (a la Shavarath) are having a nice battle over the iron tower. And they all have the pseudonatural template.
Play on their individual, real life fears. Maybe Dis is replaced by .... something
Reality changes for a few rounds at a time, perhaps liquids begin to float up, water, potions , blood
maybe for a couple rounds the whole scene is in the middle of a large city and only the PCs are/were killing people, now during those rounds, the battle is still going and the demons or whatever attacking the PCs are seen a people 'talking' instead of melee, 'dancing' instead of casting a spell etc... then the PCs are whisked back to hell to discover that the 'nice npcs' were actually beating the crap out of them in the 'real' demonic dream/flashback world
Have you ever had a nightmare where you are fighting people, but you cant punch hard enough to hurt anyone? Have a few god-mode enemies that are unexplainably tough(invincible to anything the pcs can come up with)
end the session with pcs in deep crap
during the following session, pick up as though the halloween session never happened. In game and in real life. If you can keep a straight face,...
EDIT: dis is replaced by an innocent, invulnerable collossal kitten, pouncing on combatants as though they were tiny insects, including the pcs