Roby concocts a complicated story in 'Popular Traditions of Lancashire' (1843). You can't help thinking you'd be better off waiting for the film version. But I'll try to summarise.
The story starts off with three rich men out riding with their servants. One of them is a Pilkington, from Rivington Hall. An awful storm blew up, and they decided to shelter / watch it from the tower on Rivington Pike. Amidst the eldritch thunder and lightning they heard a bang on the door - all the dogs cowered but one of the men, Norton, opened the door (cue creaky hinge noises). Outside was revealed the silhouette of a gigantic dark-dressed figure wearing a low browed hat, sat on a horse. Everyone else was scared to death, but Norton seemed to recognise the stranger - 'it's my uncle, who disappeared twelve years this very night'. Whaat? Everyone else looked on in confusion as he galloped off with the terrifying figure.
Pilkington was weighing up whether to follow his friend, but one of the servants warned against it.. the Spectre Horseman.. it must be ten, no, twelve years since my father encountered him.. he went out poaching, it was the same night as tonight, St. Bartlemy's Eve. The dogs came back without him, they stank. I went out to look for him, I was terrified he'd got stuck in a bog. I had no luck, I returned for the dogs, but then my father turned up at the house in a right state - 'I've seen th' ould one'.. A man on a black horse had stopped him on the moor - "Can'st thou show me to the Two Lads?" he said. 'My father began to wonder what this unlikely thing could want there at the Two Lads, which as you know is on the highest and ugliest part of the whole commoning; a place which is always said to have a bad name sticking to it.'
Having got there, he was about to leave, but the strange man asked him to stay - 'Now, lift up that big heap of stones there, and I'll tell you what to do with them.' 'Sir,' says my father, 'You are in jest.' But not a bit of it - the other smacked them with his horse switch, and up they jumped like crows from a corn-field. The dogs started howling and turned for home, and father was left with the Spectre Horseman that was always said to ramble about these hills, sometimes in the air, sometimes on the ground, without ever a footprint. Where the stones had been there was a great gaping hole, and horribly, a great long black arm came thrusting out of it. 'Take what he gives you!' came a voice like thunder. But father couldn't move. 'Hurry or I shall miss my time!'
But suddenly there were the sound of steps through the heather and the horseman looked more cheerful - 'Go, fool, here is one better than thee', and he kicked the poor man out of the way.
To cut this excessively long story short, Pilkington and the others decided to head up to the Two Lads to see if they could save their friend. They found him in a terrible condition but alive. Norton explained how when the mysterious horseman had turned up, he'd felt under the influence of a dream, convinced that the figure was that of his uncle, who had gone missing long before. The story gets a bit vampirey or bodysnatchers or something, as though the Fiend has to find himself a new body every twelve years. But somehow Norton manages to resist, and the Spectre Horseman appears no more. So you should be safe up there.
But the book suggests that the story's not entirely newly made up - that The tradition prevalent in the south of Lancashire ... was that a dark gigantic rider, upon a steed of vast dimensions, was wont to traverse in stormy nights the hills of Horwich Moor, and the usual spot of his disappearance [was] one of those monuments which we call Druidical, for want of a better name.
Btw, it seems that St Bartholomew's day was the day that autumn began. So that's probably why the devil was in a hurry. But he shouldn't have left everything to the last minute, should he.
There's a link at the Darlington and Stockton Times that's quite interesting.
Mr Rasbotham, a Lancashire magistrate in the last century, describes the ancient monuments called the Wilder Lads, as they existed in 1776:
Upon the summit of Horwich Moor lie the Wilder Lads, two rude piles of stone, so called from the popular tradition of the country, that they were erected in memory of two boys who were wildered (that is, bewildered), and lost in the snow at this place.
They may be seen at a considerable distance. They are undoubtedly of very high antiquity, and were originally united by a circular mound, above three quarters of which as yet remains visible. Their circumference is about twenty-six and a half feet, and the passage betwixt them six and a half feet.
From Blackwood's Edinburgh Magazine vol XLI, 1837 (p752).