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A Halloween-festive choice from Giffen is this story from GHOSTS 106, "To Kill A Ghost", a 9 page story of a sword-fencing champion who visits Scotland, and has to contend with locals who still regard a mythologized 14-century feudal lord as the greatest swordsman of all time. And staying in the deceased lord's castle, the fencing champion comes into conflict with the former lord himself. It has an unusually upbeat conclusion for a ghost story.
https://viewcomiconline.com/ghosts-issue-106/

This was one of several shorter backup stories Giffen did in late 1981-1982, because he'd leftt DC abruptly in 1978, when he came back in 1981, they took him back, but on a sort of low-profile probation trial basis to prove his reliability on these shorter stories, for :
GHOSTS
104 ("What Makes A Ghost"), 106 (above) and
111("The Last Kung Fu Movie").

HOUSE OF MYSTERY
284 ("The King and the Dragon") and
301 ("The Choice").

THE UNEXPECTED
219 ("Dust Devil") and
222 ("No Penny, No Paradise").

WEIRD WAR TALES
124 ("The Window")

Paired with a nice spectrum of inkers, and I think they determined that Larry Mahlstedt and Bruce Patterson were the best inker choices for Giffen.
Even Giffen's breakout work on "Dr Fate" in FLASH 306-313, and in LEGION 285-286 began as backups, before Giffen started doing full-issue stories.

All of which is a nice sampling of Giffen mystery stories for Halloween.