"Come on, Aziraphale, we're going out!" Crowley told the angel, waving two tickets at him gleefully and flopping down into an armchair.  "These are tickets to the gig of the century: all the best bands gathered together in one stadium to play! They're coming here from all over the world!" The demon was practically shivering with excitement.
Aziraphale smiled benevolently at his enemy/colleague/sort-of-friend and surreptiously flicked some dust off the sweater he was wearing. "I see, my dear. I suppose that-"
"Uh-uh, let me finish," cut in Crowley, grinning sharklike at the angel as he laid down his ace in the hole. "All proceeds go to charity," he finished, folding his arms triumphantly. "It's practically your divine duty as an angel to attend."
"So how do you plan to explain your presence there if anyone asks from your side, my dear?" Aziraphale asked, putting down his cup, teaspoon and saucer very carefully and s moothing down the tablecloth. 
Crowley scoffed. "Please, like they'll even notice Down There. Anyway, I'll just say that I wanted to corrupt all the well-intentioned schmucks again. Works every time, I tell you. And--"
There was a brief, disorienting moment of speed, a brief smell of sulfur, and then Crowley blinked and looked around. He was in a dark,and dingy room. Oh, no... not a summoning... He groaned and turned around to see a pale, dark haired man wearing... was that leather armour...? and wielding a pointed metal staff.
"Tremble, spawn of the deepest pit of hell," he intoned. "For I am Loki, the most powerful of them all, and I command thee to do my bidding."
Crowley looked down to see a summoning circle. It was well drawn, but fairly useless for holding him captive. He raised an eyebrow at this 'Loki'. "No puny mortal hasss the power or sssskill to bind one of the truly fallen. I will not sssserve thee. " he replied, playing up the hiss for all it was worth.
 And tartan.
 Crowley was largely of the opinion that if some idiotic satanists wanted to spout Ye Olde Englishe garbage at him then he would jabber it right back at them and see how they liked it.
"Thou must do my bidding, insolent serpent!" The boy  spat in fury, his face darkening. The dark shadows beneath his eyes made his eyes gleam in a way that unsettled Crowley slightly. "I command thee! I am your master!"
The serpent gritted his teeth in irritation. This kid was really starting to annoy him now. It was lucky for the little rat that Aziraphale was so against him wishing people out of existance, that was certain... Still, he could put the wind up him a little...
Crowley whipped off his sunglasses and glared at him with golden slitted eyes. "Sssserpent you call me, and ssserpent I am: the firssst, the tempter. I answer to none but the ruler of Below, the Adverssssary." he paused. "If thou wissssheth to gain wealth, power or knowledge, thou may ssstrike a deal with Hell, sssummoner."
Loki frowned doubtfully. "What kind of deal?"
Crowley grinned, sharklike. "Oh, the standard deal: you can have incredible power, wealth, or just the ol' 'long life and happiness' deal. Your choice, really. Anything you want, within reason."
"You... sound different than before." the Jotunn ventured, suddenly uncertain.
"Well, if you drop the stupid 'thou', 'thee', 'spawn of satan' nonsense then I will too, and I'll knock off the bloody hissing, yeah?"
Loki raised an eyebrow. "Agreed. And in return for these boons... m y soul?" his eyes glinted with hidden mirth.
Crowley snorted, slipping his shades back on. "Yeah, right. You're immortal, genius. That's what we in the game call 'cheating'. We could wait for a thousand years and never see so much as a glimpse of your soul." He waved a hand lazily and brushed a speck of dust from his impecably tailored suit. "Twisted and rotten though it may be. No, you can just owe me one."
Loki blinked. "What, owe you a soul?"
Crowley sighed. Idiots. Idiots everywhere. "No, no. You'll owe me a favour, and I'll put you in touch with a war hungry alien army, ready to obey your every command. Seem fair to you?"
Crowley felt slightly smug: he didn't need to use any miracles to put the two sets of aliens in touch, so it wouldn't show up Below, and Crowley got to keep the favour all for himself. Who knew when having a god on your side would be useful? 
"Agreed." the trickster god replied, smirking slightly, and extended his hand to seal the deal.
 Speaking as an entity that had existed in one form or another since the creation of the Universe, Crowley viewed any living creature younger than 2000 years old as a child in comparison to himself. 
 This did, eventually, lead to the feeling that he was stuck in one massive cosmic nursery s chool with a co-worker who spent most of his time fingerpainting in the sandpit with the rest of the toddlers.
 'god', mind you, not 'God'. The essential capital letter made the crucial distinction between:
1) A commendable effort at the manipulation and corruption of powerful and influential beings, and
2) a good and righteous smiting from the creator of the universe.
Crowley clasped his hand, infusing the touch with a dab of Power to make the Contract binding.
Loki gasped as a tendril of dark smoke wound its way around his hand and up his forearm. It left an unpleasant-looking demonic sigil there, which glowed red for a second before fading into the skin.
"Just a little something to make sure you don't renege on our deal, hmm?" Crowley told him. "It's a little difficult to trust the god of lies. No offense meant."
"None taken." Loki paused. "I assume you'll need help getting out of the circle."
"Hmmm? Oh, no. I'll be fine by myself, thanks." Crowley replied with a smile, stepping out of the circle with ease.
Loki drew back in shock. "But... the... the circle! It's meant to bind you there!"
Crowley snorted again. "Please. This couldn't even hold an imp, let alone a fallen angel. The circle only summons, it doesn't trap me. I could have gotten out at any time I wanted. Lucky for you I decided to cut a deal." He smirked, giving him a nod. "See you around."
The demon snapped his fingers and disappeared in a puff of sulfurous yellow smoke.
~ GO ~ GO ~ GO ~
Two weeks later, when the television showed alien invaders trashing New York and Aziraphale was tutting disapprovingly at him, Crowley was lounging in an armchair and smiling smugly in the knowledge of a job well done.