There are many theories as to how this seemingly innocuous playing card came to be known as a curse to the Scots, but I like the one offered in the following passage from my paranormal WIP, THE QUEEN OF SWORDS. In the scene, Graham Logan, the hero, is recalling a long-ago conversation with his grandfather about the Battle of Culloden, which the old man witnessed as a wounded officer of Bonnie Prince Charlie's. At this point, he's just told Graham how the MacDonalds refused to charge with the rest of the Highlanders because of a perceived slight:
"Well . . . the MacDonalds let their pride go to their heads that day. And God swiftly smote them down . . . taking the rest of us down with 'em. Our brave lads fell in writhing heaps--the forerunners skewered, slashed, and gutted by English bayonets." He shook his head. "Poor devils! May God rest 'em. But they met their end as brave men should: fighting for what they believed in, toe-to-toe with the enemy." His granda paused then, released a heavy sigh, and bit his lip, overcome. After a moment, he began again with a strained voice. "We lost twelve hundred good strong lads that day . . . "
Graham gulped at the number of dead. "D'ye think ye would have won if no' for the MacDonalds?"
"No, lad. For the deck was stacked against us long before we took the field, I'm sorry to tell ye--starting with the card upon which the Butcher scribbled 'no quarter' at a gaming table the night before the battle." He turned to his grandson with sorrowful eyes. "D'ye ken what it means to give such an order?"
Young Graham's heart was heavy with the answer. "It means that none can be taken prisoner . . . that all survivors must be kilt."
"Aye." The old man shifted his gaze to the dying fire. "And the barbarity that followed is too gruesome for me to burden ye with at yer tender age. For now, let me just say that there's good reason the Nine of Diamonds will forever be known as the Curse of Scotland." He sat in silence for a moment before adding, "Now off to bed with ye. And be quick about it."
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For more theories about how the Nine of Diamonds earned its ominous name, check out this article from The Scotsman.