You are all sitting in a wonderful tavern enjoying the anticipation of the coming holiday! Yeah, you can sense there are others there like you-avatars of your deity. You know there are a few diametrically opposed that you would just as soon slaughter in the name of your own respective faith, but it's Christmas...you can give them a pass this time.
A gaunt and scratched up elf enters. He walks up to the counter and says to the barkeeper,
"Gimme a shot of the strongest stuff you got! Make it a double...no make it a triple...JUST GIMME THE WHOLE BOTTLE!"
Shaking, he sighs and slams a few.
"It's happened! Revenge has been served everyone! Treerazer himself has possessed The Jolly Old Elf and thrice hexed him. Believe me when I tell you this...Santa Claus is Coming to Town!!!"
He pulls a red sack out of thin air, "I was able to get away with this, but I can only use it to a limited degree!"
He takes another shot...or two,
"The first goal is to end the overt possession itself. No, you won't be expected to fight Treerazer himself at first level-Santa was able to retain enough Christmas cheer to prevent the demon from physically wreaking havoc-but he will release the Twelve Curses of Christmas. For those that survive the tournament of death that is the Twelve Curses, you will gain a level after each curse is beaten...that's the first stage!"
He takes another shot...or three,
"After the Twelve Curses are complete, and Treerazer's physical manifestation is exorcised, you will be left with a super-lycanthrope! Treerazer sent a demonic werewolf on a suicide mission to attack Santa...we killed it-but not before it slaughtered all the reindeer and bit Santa, obviously! Although conventional cures will help your cause...you will have to kill it. tThat is the second stage"
He produces Silver Bells from Santa's Sack, "With these, I can lace each of your deity's favored weapon, or ammunition in a silvered fashion for the duration of the stage. While they are silvered, though, all magical properties of the weapons and ammunition will be suppressed!"
He slugs the rest of the bottle,
"That will leave you with the third stage...A Mega-Zombie. It's not your typical undead creature! By this time, with the magic of Santa's Sack, I will be able to bring back anyone who died in the early stages, but only at the strength you were slain at. Once you defeat it, Santa will be returned to normal!"
You all look around, and notice the tavern has gone from a bustling and cozy establishment to a barren and run-down cantina-much like you would find in a ghost town abandoned for at least a century. The entrance doors glows red...and opens.