Disclaimer: I do not own Good Omens and I make nothing from this except my own entertainment.
1703 AD, Lyons
Thomas de Meschains was a man on the edge.
On the edge of a breakthrough, that was. He'd spent years poring over every detail of the Clavis Salomonis and the Clavicula Salomonis and the De praestigiis daemonum; he'd studied the Pseudomonarchia daemonum until he could have reeled off a list of every demonic spirit named therein, and had in fact developed a pronounced tendency to mutter infernal titles and powers in his sleep; and he was running out of space for his collection of ritual rings. He fasted so often that standing up made his head spin; and if there was a less polluted man in France, he'd probably washed himself out to sea in a stream of soapy bubbles. There wasn't a thing Thomas didn't know about demonic seals. He'd drawn so many chalk circles that a groove was developing on his workroom floor and only one thing was still missing...
... a demon.
Any demon. It really didn't matter which.
He'd started small, of course. Furcas and Bifrons had proven frustratingly indifferent to his summons and Thomas hadn't had any luck with any of the other infernal knights either. Once he'd exhausted the list of prelates and presidents, he'd taken a chance on the more powerful spirits. He hadn't got anywhere, but at least it hadn't been much of a surprise this time. In fact, when frustration finally drove him to tug at Baël's metaphysical bellpull, the continued emptiness of the summoning circle came as a bit of a relief. Thomas wasn't sure he wanted to deal with the power of the east and the principal king of hell just yet.
Anyway, Thomas was pretty sure he knew what he'd been doing wrong now. And it had been complete chance that he'd run across that nice young man with the odd eyes selling off his grandfather's grimoires in Marseilles!
Not that the young man had known what he'd got, of course. A couple of convivial drinks in an inn's common room had resulted in an invitation to take a look at the crumbling and unintelligible tomes inherited from a deceased progenitor. Thomas hadn't been expecting anything more interesting than a few battered Roman classics, so he'd had trouble keeping the surprise from his face on recognising a decidedly antiquated version of the Ars Goetia. And then he'd seen it.
The fountainhead. The wellspring of knowledge. Johann Wierus had known of it, and read it, and copied from it – but incompletely. How often Thomas had cursed Wierus's unscholarly scruples! the omissions made 'lest anyone who is mildly curious may dare to rashly imitate this proof of folly'! Any would-be conjuror was obliged to fill the gaps as best he could, in Thomas's case with a marked lack of success.
And here it was. The Liber officiorum spirituum.
He couldn't even begin to guess at how old it was. It had been written on parchment. The script was scrawny and unfamiliar and nothing like the thick black lettering that filled Thomas's own grimoires. Of course he'd told the young man it was a curio of slight interest to a serious collector only; and then he'd paid a fraction of the book's actual value and spent the next few months deciphering the horrible scratchings of some long-dead scribe.
He'd tried the summoning circle again. At last, Thomas was sure, he'd almost broken through. A whiff of brimstone had lingered in his workroom for days. He'd gone back to the Liber officiorum and reread everything and made several dozen corrections and had another go. Brimstone and red light had resulted; but still no infernal spirits. And now he was as sure as he could be that he'd got everything right and it was time to make a third attempt.
This time, Thomas was going to be ambitious. He'd selected his demon and waited for a nice clear morning, just like all the books said. The chalk circle glowed white against the floorboards. His embroidered robes had been cleaned specially and he'd hunted up his finest ritual ring for the occasion. The mirror whispered reassuring things about his appearance. It wouldn't do to meet a duke of hell and not be smart.
By now, the words tripped off his tongue. He gripped the edge of his lectern and peered intently into the circle...
1989 AD, A Certain London Bookshop
"... and Hastur jumped out right behind him," finished Crowley. "Serves the idiot right. I'd say it taught him a lesson, except it was a pretty terminal one. Hastur'd been waiting for him to get the words right for so long that he wasn't in the mood to play along any more."
He reached for the bottle. "Hey," he added, discovering that Aziraphale had polished off the last remaining dribble, "got any more of this?"
Aziraphale produced more wine. "Poor fellow."
"Poor fellow nothing!" said Crowley. "Anyone stupid enough to summon a demon outside the circle deserves a sticky end. And," he went on reflectively, "it was pretty sticky, as I remember. Must've been a hell of a job cleaning up afterwards. Funny thing is, there was rather a rash of stupid conjurors in that area after that. I think the next couple that got hold of that book did exactly the same thing, which is odd, because there wasn't anything wrong with the book, I wrote it myself..."
They drank in contemplative silence for a moment or two. Then Aziraphale set his glass down and said, "When did you write it?"
"Sometime in the tenth century, I think. Why?"
"Ah," said Aziraphale brightly, "then I think I can explain your conjuror's little problem. Per. And probably also autem. Yes, most definitely autem, given the nature of the mishap."
Crowley stared at him. "What?"
"Unless I am much mistaken," said Aziraphale, "you were writing in Visigothic script. You spent most of the tenth century in Spain, didn't you, dear boy? Visigothic does have some odd little quirks. The usual abbreviation for pro means per, for example. But I think your conjuror must have been most confused by that nice young Tiro's horned H symbol for autem, which does look so distressingly like an abbreviated hic if you don't know any better. It's quite a common mistake."
"Is it?" said Crowley, blankly. "So?"
Aziraphale, given the opportunity to discourse on a favourite topic, was developing a smug sort of glow. "So when your conjuror said hic, which is to say 'here', instead of autem, which is to say 'however', he must have inadvertently summoned Hastur outside the circle. You see –"
Crowley recognised the signs and reached blindly for the wine. Some things were not meant to be discussed sober.