Warnings: this is the scary chapter. I've tried to make it as dark and atmospheric as possible, so do let me know if I've succeeded.

Oh son, who would leave a child in this place
To the wind, and the dark, and the rain on their face?
The rain on their face, and the mud on their feet
What kind of a king would dress his children in leaves?

Josh Ritter, 'The Oak Tree King'

Elsewhere:

The Bavarian Forest National Park exists on the border between Germany and the Czech Republic. It is comprised mostly of spruce, fir and beech trees, with some bogs scattered here and there. Popular with tourists, on this particular evening, it was misty and cold and both tourists and the locals had eschewed the park in favour of warm dinners and comfortable beds.

Deep in the forest, in the highlands dominated by spruce trees, where despite the spring the air was still and chilly, James Moriarty was picking his way through the trees. He moved slowly in deference to the fading light, but also moved with unaccustomed caution, owing to who – what – he was seeking out.

James Moriarty had come to Germany to find the Erl King.

He had paid an outrageous sum to find out the Erl King's last known whereabouts, and the most reliable source had said the Bavarian Forest. The Erl King was a nomadic being, but had a distinct preference for forests and woodlands, and would regularly move his court and his stolen children from one desolate forested region to another. Such movements made him difficult to trace – not that many people sought him out.

But Moriarty had a purpose in doing so. He wanted something – something only a being as old and powerful and cruel as the Erl King could give him.

He was being watched – he had been trailed from the moment he entered the forest. He whistled merrily, waiting for the moment the Erl King's minions would make their presence known. Probably they were debating whether they were hungry enough to bother with hunting and killing him.

Let them have their fun – Moriarty would have his in due course. He giggled at the thought. It had been ages since he'd had some mindless enjoyment, since he'd made something squeal and shriek just for the hell of it. His torture chamber (yes, he actually had one) had been used for either interrogation or punishment lately. When this business with Sherlock was over, he'd snatch some innocent and do a little defiling, a little tormenting... nothing too demanding or creative. The fun he could have with a simple metal spoon and some pliers...

He lost his train of thought as the first demonic creature came at him. It was winged, had leathery skin, sharp claws and teeth and was more than a match for most men. But Moriarty, made powerful through years of deals with demons, caught hold of it mid-leap, snapped a wing just for the hell of it, enjoyed the thing's yelps of pain for a moment and then wrung its neck.

Moriarty tossed the corpse to the ground, then grinned his maniac grin at the surrounding forest, all shades of blue and purple in the twilight, at the eyes it contained, wide and pale and bloodthirsty.

'Tell daddy-o that there's a magician here to see him,' he called out cheerfully. 'A really, really dark magician. And I will totally make it worth his while!'

The Erl King's court was situated in a rocky gulley, twisted trees overhanging it and blocking out the rising moon, so that the place was in darkness except for the torches stuck in the ground at odd intervals, which glowed with an unearthly bluish light that lent a pale, sickly look to everything they illuminated.

Including the Erl King himself.

The Erl King was gigantic. He stood nearly eight feet tall, and would have been taller but for a pronounced stoop, suggestive of his great age. His features seemed hewn from craggy rocks, his skin was tanned and leathery, his hair was frosted grey rimed with greenish moss, he was dressed in tattered animal skins and the bark of trees, and the bones of various unfortunate victims, both human and animal, decorated the long fur cloak he wore. A crown made of oak leaves and thistles adorned his head, and his eyes were black as the river at night, but colder and with an ancient malevolence lurking in their depths.

Around his feet and the hem of his cloak, children gathered – how many, Moriarty could not tell. They faded in and out of his view, mingling with the shadows and the night. They were dressed in rags and in autumn leaves, they were spider-leg thin and gangly, their skin pale and clammy, their feet muddied, their eyes vacant and mournful, tormented by memories never quite forgotten – of a warm hearth, home, family, a love that was not twisted by possession or domination. Yet they clung to the Erl King as their only means of comfort and protection in the dark, ruthless realm that was his kingdom.

In the darkness lurked his court. The meaner, nastier kinds of Fae, various demons and monsters, an occasional human, seduced or kidnapped into service... They were an ugly bunch, easy and violent, lusting to be tampered with and to exact payment in blood and misery. But Moriarty had no time for any of them.

For the Erl King's black eyes, the colour of mould and rot, looked upon James Moriarty, looked through him and into him, and for a moment even Moriarty, psychopath, sadist and veteran in evil, quailed before the wickedness they contained.

Then his sharp sight sought out the Erl King's left arm, tucked close against his ancient body. It was blackened and withered, marked by unhealed wounds, crippled and nearly useless. It looked almost as if it had been struck by lightning, or gashed by the talons of a striking hawk.

Moriarty sucked in a deep breath, impressed despite himself. The work of Cerridwen and Violet Holmes, there was no doubt about that. Briefly, he wondered what they must have traded in to gain such power. It couldn't have been innate – the Holmes women only ever wielded small, simple magicks, to do with the weather and birds and such nonsense. But it was proof that the Erl King was not invincible – he could be injured, outwitted, outmatched.

Confidence restored, Moriarty met the gaze of the Erl King boldly.

'You sought me out,' the Erl King said slowly, voice sounding like the creaking of dead wood, like the fierce winter wind, like ice snapping underfoot. 'You have found me. What would you ask of me, dark magician?'

Moriarty grinned. 'A while back, you had a little contretemps with a magician named Violet Holmes and her sister-in-law,' he began.

The Erl King drew himself up to his full height, face contorted with rage, as around his feet the children scattered, terrified.

'That name is never to be uttered in my presence!' he said, and though he never raised his voice, Moriarty could feel his rage, like a million tiny wasp stings, like poison seeping into every pore. He knew he was mere moments away from death, or far, far worse. It was rumoured that the Erl King was on intimate terms with Old Nick, and had been known to toss him a hapless soul or two, that he had been known to keep prisoners alive for years, simply in order to inflict as much pain on them as possible.

Brilliant!

'Actually, it's not her I'm bothered with,' he drawled, sticking his hands in his pockets in careless fashion. 'It's her youngest son. Sherlock. Quite the curse-breaker, a talented little magician. No wonder you took an interest in him, he's getting very powerful.'

The Erl King's eyes narrowed. They looked like hard little chips of onyx in that leathery, furious face.

'He was glorious – so strong. So gifted. He would have been a prince among my children,' he sighed, a sound akin to the rising of a hollow wind. 'As would his brother. Had those cursed women not interfered – he is of no worth to me now.'

Moriarty raised an eyebrow, a carefully calculated gesture. 'Worthless? Perhaps. But I wouldn't write him off just yet. We're enemies, him and me. We've been going at it like rabid dogs for years. He's the greatest opponent I ever faced.'

He eyed the Erl King, but that weather-beaten, craggy face betrayed nothing, in stark contrast to his earlier fury.

So Moriarty wittered on. 'But just recently he's gotten a teensy bit annoying. I cursed him a few years back, you see. One of my best creations, powerful and enduring, and the amount of pain it's inflicted was impressive even by my standards.' He let loose his goblin giggle at the idea, but turned serious again in an instant.

'But he's worked out how to break it,' he confessed. 'He has to fall in love and be loved in return. The solution is all about how his loved one sees him, and how he sees himself. And, regrettably, he's managing it. He's fallen for some doctor who turned up on his doorstep.' He allowed his bafflement to show on his face – if Sherlock fell in love, fair enough, but with the stunningly ordinary John Watson? 'Pretty disappointing, the good doctor's as dull as dishwater, no spark of genius or grandeur about him. I'd always believed Sherlock was destined for great things, but now –'

'Is there a point to your inane ramblings or should I just behead you and spare myself the tedium?' the Erl King interrupted, bored. His minions and his court edged closer, sensing the potential for violence in the situation.

'Ah, yes.' James cleared his throat and decided to cut to the chase. He liked flirting with danger, but had no desire for a consummation. 'Long story short, I want to beat him. No, not just beat him – I want to burn him. Burn the very heart out of him!'

The Erl King said nothing, but Moriarty thought he saw the faintest flicker of interest across that prehistoric countenance.

'But for that, I need power.' Moriarty paused, before delivering the coup-de-grace. 'I need an Unbreakable curse.'

The court drew its breath as one.

A great silence descended; settled; lingered.

Moriarty waited, until the Erl King spoke.

'And why should I bestow such power upon you?' the Erl King asked him coldly. 'You are a little, petty, meddling conjurer who steals power or bargains for it and uses it as a mere toy to amuse himself! There is no true magic in your blood or spirit. Your enemy is unquestionably your superior.'

Moriarty offered no riposte to the insults, keeping his stance casual, but those strange, uncanny creatures lurking closest to him saw his hand gripping at the hem of his coat pocket so tightly the knuckles turned bone-white.

'You should bestow it because it will give us both what we want,' he drawled in response, tilting his head back and looking at the Erl King from under lowered eyelids. 'I'll burn the heart out of my rival, and you – you'll have vengeance for what was done to you.' He gestured at the Erl King's damaged arm, and saw the Erl King's stolen children gasp in horror. He guessed he had just violated the ultimate taboo of the Erl King's court.

'Think about it,' he continued, speaking rather more hurriedly than he had a few moments ago. 'The greatest revenge, the greatest hurt you could inflict – not targeting the one who did you that injury, but going after the thing she loved most in the whole wide world. What could offer greater vengeance than going after her son? I'll throw in Mycroft Holmes as well, if you like. For free! Oh, and I'd like a little residual power to heal a lieutenant of mine. He ran into Sherlock recently and was blinded – light magic, you know.' He rather enjoyed the stir that little titbit caused amongst the courtiers.

The Erl King, still impassive, scrutinized Moriarty carefully, those hard little eyes taking in every detail on his inside and his outside. His gaze stripped away flesh, blood, bone and he looked upon the man standing before him, seeing his insanity, his obsession, the thousand little spiteful cruelties that occupied his mind at any given time, the lengths he would go to in order to gain victory over Sherlock.

The Erl King smiled toothily. It was a fearsome sight.

'And if I give you that power,' he said, 'what will you offer in exchange?'

'Well, I already traded in my heart,' Moriarty wittered blithely. 'But all my other body parts are up for grabs!'

The Erl King shook his head. 'No, enemy of my enemy,' he whispered harshly. 'I do not want your limbs. But I will have your soul. Grant me that, and I shall give you the power you so desperately crave.'

Moriarty did not even hesitate. 'Done.'

The Erl King laughed softly, a sound like the rattling and crumbling of old bones. 'When shall it be done? Tonight?'

'Soon as you like,' Moriarty said carelessly. 'Just give me five minutes to make a phone call, would you?'

The Erl King dismissed him with an imperious gesture. Moriarty turned his back on the jabbering demons and stolen children and pressed speed-dial on his mobile. A handy little spell on the device meant that it was never without a signal.

'He's agreed to the exchange,' he said without preamble as soon as the person on the other end picked up. 'Full steam ahead. You know what to do. She'll leave the meeting at nine UK time, and she'll walk home, you'll have plenty of opportunity. And if you want to keep the skin on your back, make sure DI Lestrade is informed of what takes place.' He listened contemptuously to the murmurs on the other end of the line before laughing dismissively. 'Leave Mycroft Holmes to me. I'll make sure he doesn't get back to England for a while yet.'

Then he hung up – and caught sight of something out of the corner of his eye.

He whirled round, ready to rip whatever it was asunder and drink its blood, but saw nothing threatening or even living. It was only the watery, uncertain reflection of the moon, shining down on a little bubbling brook, on its way to join one of the rivers running through the land. Nothing more.

Moriarty looked at the running water for a long moment, then shrugged and went to relinquish his soul for all eternity.

Author's Notes: beware, Sherlock and John's happy time is not going to last for much longer... till next time, dear readers!