Well, pick up that phone, and call Honest Uncle Grimbo's Party Hire and Undertaker TODAY!
He does weddings! Birthdays! Bar mitzvahs! Costume parties! Spooky parties! And he makes one heck of a mean Ridgeport!
Hula Zombies not included.
.....yet.
What this mod does:
This modifies the "bartender_CostumeParty" xml file, and changes it so that when hiring a Mixologist for your Costume party or Spooky party, the Grim Reaper will show up as the Mixologist. It also changes his job title from "Soul Reaper" to "Mixolgist" in these situations. Because this is his day job. Reaping is really more of a hobby.
You don't need Spooky Stuff for this mod (as far as I know), as it appears that Costume and Spooky parties draw their bartender from the same file. Additionally, it should be noted that while Grim will show up for work on a community lot with a bar, the game automatically assigns Mixologists on Nightclub and Bar lots, so he'll mostly just act like any other partygoer in this situation.
What this mod DOESN'T do:
For any other party, this shouldn't affect it. I've tested this with a House Party and a Dinner Party, and I got an ordinary Mixologist instead.