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This came up in on of ligedog's games that my youngest was in at last week's Gary Con.
What does a slow spell do to a flying griffin?
There was some discussion about their wing speed dropping such that they could no longer fly.
I think ligedog however ruled about what I would (this is my interpretation of what little I overheard).
I would rule their speed is cut in half per the spell description.
How would they react to this? Well, that's up to the DM and I think ligedog handled that better than I would have. That is, some sorta freaked out and glided to safety, and one continued on doing what it was gonna do.
Just something I hadn't thought about in the years I have been gaming.
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I agree with that ruling.
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I also would rule that it simply cuts their flight speed in half. Griffins generally aren't depicted with wings large enough to realistically lift a muscular, solid-boned creature of their size anyway, so their flight must be at least partially magical.
The original intent of the slow spell clearly wasn't to be an instakill flying monster spell.
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Yep, I was even thinking that griffins and the like probably get some of their flying from magic. However, I would still rule the same for non-magical winged creatures. Well, maybe not laden swallows regardless of origin. They're bastards and deserve to plummet to their death if they're trying to import coconuts.
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I had definitely never seen it before! A good use of a spell and totally foiled the swoop, grab, and drop from great height tactics (the party was standing atop a structure halfway up a steep mountain) the griffons had been planning on.
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Interesting player ploy, but I like the ruling, too: it halves movement rate and attack rate, and explicitly doesn't affect sorcery.