It did not take long for Mrs Hudson to work out that Merlin and Sherlock were perhaps not the perfect pair of flat-mates. It wasn't that they didn't get on: they didn't interact with each other much, but it was clear that they managed not to tread on each others' toes. Nor was it that they got on... well, too well. No, it was more that they were both hopelessly incompetent in the normality department.

Sherlock stayed up all night living in his own thoughts. Merlin had a tendency to walk the city at odd hours, sometimes from dusk until dawn. This meant that both of them would sleep in late in the mornings, and Merlin would find himself running to get to his lectures whilst Sherlock sleepily awaited Mrs Hudson's breakfasts. (She had said many times that she wasn't their housekeeper, but seeing as they were both incapable of remembering to eat breakfast, she had to give them a bit of a nudge in the right direction.)

Both of them could cook. Neither of them did.

Sherlock owned a gun. Merlin apparently owned something that could wreck wallpaper in a similar fashion, though it must have been silent, because Mrs Hudson never heard it.

Sherlock spent hours agonising over the details of his latest case – apparently still the locked-room murder, which he didn't seem to have solved yet. Merlin spent hours agonising over some ancient mouldering literature specimen.

All in all, it was a bit odd.

The one thing that never occurred in 221B Baker Street was conversation. Yes, if Sherlock and Merlin crossed paths they would acknowledge each other – remarkable coming from Sherlock, perhaps, but merely an indication that he didn't dislike his flat-mate. But nor did they ever speak to each other. It was beginning to look unlikely that they would never truly know each other.

That is, until one particular Wednesday afternoon, when the silence of the flat was broken by a sharp bing.

Sherlock, who hadn't been expecting a text, sat up in surprise and reached for his phone, glancing over at Merlin, who was still poring over his dusty old book.

The Strand. Similar murder to last time. Writing on wall. Come asap. DI Lestrade

His curiosity piqued, Sherlock went and flung his coat over his shoulders. Merlin looked up.

'A case?' he asked.

Sherlock nodded. 'Hopefully one that will bring me closer to solving the other one.'

And with that he went from the room.

There was already a cohort of police cars blocking off a large section of the Strand, and tape all around one of the houses. Sally Donovan was standing outside this house, and cast a scathing glance towards the detective.

Just as he was crossing the threshold of the doorway, she turned to him, and called out:

'Isn't that guy you're sharing with called –'

And thinking better of it she grimaced and turned away.

Sherlock didn't bother asking her what she had been about to say, though he yearned to know. He heard footsteps on the floor above him, raced upstairs, and found Lestrade and a couple of others standing around a body. This one was face-up, and from the marks on the clothes it was evident that this corpse had been burnt in a similar fashion to the last one. There was also caked blood around a wound to the head.

But it wasn't this that drew their attention. It was the silver letters on the wall, spelling out a word that made Sherlock stop in his tracks.

Merlin.

He caught something about Arthurian legend from one of the police officers who had followed him in. It was certainly a strange word to daub on the scene of a crime. A name, indeed. The name of a wizard of ancient legend – and Sherlock's new flat-mate.

He quickly dispelled the image of Merlin Ealdor from his mind, and called out to Lestrade, demanding the basic details of the case. It seemed that it was almost exactly the same as the last one – the popular theory was that it was the same murderer. There was no solid evidence, but few believed otherwise. Already a couple of forensics officers were studying the body, the writing, the room, hoping to find more similarities with the other case.

Sherlock walked around the body several times. It belonged to an unremarkable man. His eyes traced a winding path around the room. Old. Few friends. No relatives. Lonely. It was this succession of words that began to prey on him. This man had been vulnerable, certainly. An easy target. But why such a violent murder? And why that name, daubed across the wall in huge silver letters?

He straightened, cast a glance towards Lestrade. It was obvious that the detective inspector was a little perplexed. Perhaps he realised that the other case was still unsolved, and that this one presented the same difficulties. Here, that word would probably be the only thing to go on.

He looked at it again. Merlin. Capital letters. So straight they could have been a computer font. Done out in some bold substance – acrylic paint? No – there weren't any drips on the –

There weren't any drips on the carpet. Nor did the letters run in the slightest. Sherlock furrowed his brow. If the letters had been painted, it would have been painstaking work to do it that immaculately. Criminals who painted notes usually dripped all over the place, and their handwriting tended to be terrible as well.

This was perfection. Companies would have hired this person to design their logos, had he not been, well, a murderer.

'Why Merlin?' Sherlock asked the room.

'Well,' said Lestrade, 'either it's a reference to the tales of King Arthur,' (it was plain he thought this a bizarre hypothesis) 'or –'

'– or it's a message to someone,' Sherlock finished impatiently.

'C'mon, how many people do you know called Merlin?'

'Just the one,' Sherlock said, mostly to himself. He drew his phone out and drafted a message; after a moment's hesitation, he sent it. Then he scanned the room again and waited for a response.

Why is your name on a wall in a house on the Strand? SH

Merlin furrowed his brow at this curious text. He assumed SH to be Sherlock. He straightened, knowing Sherlock to be at a crime scene. It took him more than a moment to register the actual content of the message.

'My name...?' he wondered out loud, and replied:

What do you mean? Merlin

A minute later:

House on the Strand. Your name is painted on the wall. Did you really need me to explain that? SH

Merlin ignored this rather rude reply, and said instead:

First name or second name or both?

First. SH

Probably not me specifically then. :-)

Merlin, relieved, turned back to his book.

It was hardly a few minutes later when he received a message that read:

Can you translate Old English? SH

Yes... why? Merlin

Come here and translate some then. SH

Merlin took the Tube to the Strand, and entered the long street just as one of the police vans was pulling away. He hoped that it contained the body. He hadn't much wanted to see the body. With a slight shudder, he made his way to the police cordon, gave his credentials, and was (somewhat reluctantly, it has to be said), let into the building in question.

Sherlock was standing over a sheet of paper that had been unfolded on a coffee-table. Upon it were recognisable letters spelling out unrecognisable words – unrecognisable, that is, to those who are limited to Modern English.

Merlin, however, knew them immediately. He read the message, gave a cry, and sat down involuntarily.

'What is it? What does it say?' Sherlock insisted.

Merlin composed himself, drew a breath, and said quietly:

'It says... "Merlin, I'm coming. Be ready. Remember Camlann."'

'Camlann?...' Sherlock said.

'Isn't that the battlefield where King Arthur died?' cut in Anderson, who had been standing to one side, but now looked far too pleased with himself. 'Yes, it was. Camlann. Haven't you read the stories?'

'Shut up, Anderson,' Sherlock said, but he had scarcely got the words out when Merlin said:

'No... he's right.'

Sherlock glared at both of them.

'Camlann,' continued Merlin, 'was the name of the field on which Arthur – King Arthur, of Camelot – met Mordred in his final battle. They ended up killing each other.'

'And why should this note refer to it?' asked Sherlock, who was trying desperately to hide his annoyance. His ignorance of Arthurian legend seemed painfully obvious.

'It's as I feared,' murmured Merlin, not answering the question. He looked over the note once more, seemed to shudder a little, and then ran from the room.