BtVS by Whedon and Mutant Enemy. Stargate by Gekko Productions and others. Idea inspired by Speaker-to-Customers' use of the Illefarn song portals in the Faerun/Stargate crossover 'Debt of Blood'. I'm moving the start of Stargate SG-1 ahead a few months to help things line up better.
June, 1997 – Prophecy Girl
Buffy and the gang opened the double doors to the Sunnydale High library and strode in, talking animatedly among themselves. They were vastly relieved that they'd killed the Master and that his death had sealed away the tentacle monster that had emerged from the Hellmouth. As they approached the broken desk that had killed the Master, his bones laying impaled amongst its ruins, a flash of blue energy rippled around the path of floor directly above the Hellmouth's opening.
With a *Kawoosh* sound a pillar of blue energy exploded out of the floor, utterly consuming the bones of the Master and many of the scattered books and equipment littering the area. The pillar settled out into a rippling pool of something comparable to water.
"No!" Giles yelled in frustration. "I mean... Yes, the desk was a complete loss, but I'm going to have to do a complete search of the catalog to find out which books are missing..."
"Well." Xander waved at the energy. "That looks like a portal. We could go in, or try fishing through it with nets to see if any of them survived?" The portal chose that moment to cut out, leaving a perfect circle of bare cement in the middle of the tile. "Or not."
Cordelia shrugged. "At least that didn't happen earlier, or we'd have pieces of exploded tentacle monster scattered around the room... Wait, would those have been worth money? Must've been a pretty rare demon from the way you guys reacted to it."
With a shrug, Jenny launched into an explanation of the local black market as the rest of the gang started putting their main hangout back into a semblance of order.
##
In the depths of Stargate Command, Jack scratched his head at the readouts being presented. "So, one of the dead addresses from the diagnostic program randomly connects to another Stargate, which disappears from the network a few minutes past first contact? I guess they somehow pulled the plug on it... Wait... This address would be really close to ours, right?"
"Extremely close." Sam nodded. "If you actually look at the footage, the wormhole formed on the fifth chevron rather than needing to be locked into the full seven. It's going to take sometime to decipher the radio signals we received from the other side, separate them from the interference caused by the similarity to ours, but it's definitely opening elsewhere on Earth."
Jack whistled. "Are we thinking active infiltration or some lost tech with a very low battery life?"
"No way to tell at this point. The most we can do, assuming we can't pinpoint the opening without more information, is increase the program's preference for dialing this address and those similar to it."
Jack rested his elbows on the table. "That really wasn't enough time to scramble a MALP with our current set-up. We either need to adjust that, or rig up something like a paper airplane with a homing beacon that can be thrown through on short notice."
February, 1998 – Solitudes
After hours trapped in an icy cave, Sam and Jack were surprised by the sound of military helicopters.
Jack cringed as he pulled himself up, despite the pain of the wounds inflicted by their forceful ejection from the malfunctioning Stargate. "Looks like we're about to be rescued... Or there's been a Goa'uld out there kidnapping, ahem, 'recruiting' in recent years without us noticing."
"Unlikely, sir." Sam began fishing through their packs for anything likely to help them attract attention, quickly deciding on a short range radio. "I've been carrying around one of the prototypes for the 'homing pigeon' in case I have free-time off-world. Seems that they locked on to it shortly after I turned it on – so we are, very probably, on Earth."
"Well." Jack nodded. "If the ice is any indication, you might have to model its next incarnation after a penguin."
"No, sir," Sam stated firmly as she made contact with the Antarctic rescue team.
May, 1998 - Becoming
Angelus and Buffy fought each other with swords as Acathla's portal began to open. His moves were tempered by his desire to have her alive until the very end so he could see the loss in her eyes as she and the world were swallowed whole.
Buffy's reluctance to fight the demon with the face of her lover had waned with all the destruction he'd caused, but she'd still come here to capture and not kill. She wasn't holding back - no point in risking her life - but there were many wounds that simply wouldn't be fatal for a vampire. All she needed was one good strike... At least, that was her plan before the sword imprisoning Acathla had been drawn. She'd been distracted by one minion too many and she was afraid that it couldn't be closed without the sacrifice of the summoner, whether that meant Angelus or Angel. She fought on, her guard raised for a different reason now. One massively wrong move and he'd be dust rather than a bleeding victim and the world would end anyway...
Sure enough, Willow's ensouling spell took hold too late and her Angel was returned to her. She took comfort in his arms for a few seconds as the portal grew expansively, the brownish-gray bleeding into blue. He'd dropped his sword, but she'd kept hold of hers... She struck home as the portal exploded away from them, forming a horizontal whirlpool with the sound of a massive *Kawoosh*.
In an instant, Acathla was disintegrated, with nothing left but its clawed feet.
Buffy dropped the handle of the sword in surprise and gravity pulled it out of the hole it had made in Angel's stomach.
He stared at her pale face, lit by rippling blue radiance, in utter shock. "Buffy, it's me. Its really me. Not Angelus. I didn't need to be stabbed."
It only took a second or two for her to register where she'd seen this kind of portal before as well as that it had, very firmly, stopped growing. She shook her head abruptly, grabbed Angel by the collar of his shirt and pulled him in for another kiss, completely soaking the front of her clothes in blood. "I love you," she growled. "But you're a vampire. You'll get over it."
"Yeah." He shook his head. "But it still hurts."
##
A short crashing sound had them carefully walk around the edge of the portal to see what resembled a very disgruntled mechanical pigeon blinking green lights to itself in the dust.
"Well," Buffy said. "That doesn't look like a refugee from any hell dimension I've seen, unless it was hijacked by the Borg or something. Where was the portal supposed to open to, again?"
Angel shrugged. "Something about unending torment. Can't really remember. Could be that Angelus just stopped researching when he had the basics covered. Isn't that the energy pool that ate the Master's bones?"
"Yeah, I guess it just has a taste for world-enders. I think we'd have noticed anything coming out of it last time though."
"Maybe it only lets stuff through when it's laying on its side. Like that thing, for instance."
They watched, quietly, as a six-wheeled robotic vehicle trundled out of the wormhole and waved a claw at them. A few seconds later, the portal cut out.
After taking a few steps, Buffy reached down and grabbed the sword that had imprisoned Acathla in its first life. She carefully handed the blade to Angel without taking her eyes off either machine. "I seriously have had enough fighting for one day. Is it too much to hope for that if we leave them alone, they won't do anything?"
Angel shrugged. "It's daylight so I'm not up for wandering very far myself. I also, technically, own this mansion now. Feel up for some remodeling?"