The Hyakki Yako (百鬼夜行), or the Night Parade of One Hundred Monsters, is one of the more well-known Japanese folklore tales, frequently a subject of old paintings like this one:
Hyakki Yako is a name given when all kinds of youkai gather together in a group to walk the streets. Of course, anyone who happens to cross their path was as good as dead. But there’s a belief that so long as you were carrying a protective Buddhist spell or scroll, you would be able to escape.
There is a story of just such an occurrence that happened in Kyoto. The eldest son of a high-ranking official was out for an illicit meeting with lover, when he saw a large group of people heading towards him, holding torches and carousing amongst themselves. He, of course, didn’t want to be seen, so he hid. When the mob grew closer, he realized that the group wasn’t made of people, but youkai.
Suddenly, one of the youkai exclaimed that they could smell human. The man thought he was as good as dead, but though all the youkai saw him, noticed him, none of them made an attempt on him and he was able to escape.
As it turned out, his wet nurse – rich families then kept wet nurses even after the child was grown, as a faithful servant – had sewn a protective charm to his sleeve.
The man didn’t leave completely unscathed – he was sick with fever for days – but he left the encounter with his life still in his hands, and that was considered lucky.
One could assume that during those times, people stayed indoors, fearing that they would run across the Night Parade, and even those who would travel, perhaps carried an amulet or two, just in case. When exactly the Night Parade appears isn’t clear, but according to Onmyoudo, it happens on certain days during the early months of January, February etc.
There’s also a saying that the Night Parade only appears at midnight – as so many creepy-crawlies do – because that is the exact time when yin becomes yang. So just be sure not to be outdoors during that time.
Since there don’t seem to be any exact distinctions on what kind of youkai are present during the Night Parade, this concept has one of most potential. Those who are interested should check out a manga called Nurarihyon no Mago, which features an in-depth executions of how a Hyakki Yako might operate.