Do not own BtVS.
The spell to awaken the Slayer essence in all the potentials goes differently than canon. (Character death – Including Kennedy)
Willow felt the power go through her and knew that the spell had worked. Her first indication that something had gone wrong was when she had lightly tossed the Slayer Scythe to her lover, Kennedy, and cut the girl in half.
Giles saw the Scythe crash through the wall, covered in blood and plaster.
“Giles. I did the spell. I think something might have happened though, get that to Buffy.” he heard Willow call through the hole in the wall.
He didn't pause to ask might have happened to coat the weapon in blood. Buffy was in a cave with thousands of Turok-han and needed his help. He raced down the hall to the cave entrance. The view that greeted him there was horrendous. The Turok-han had overrun whatever lines the girls had once been in. Buffy and Faith were nowhere to be seen. Violet was fighting with obvious Slayer strength, but the others were not. Rona was keeping them together, her broken arm meant that she couldn't fight as well as even a normal human, but she could shout orders like the best Drill Sergeant. She got the girls to offer a solid defense against the vampires but the girls weren't doing so hot. Two of them were pulling an injured Spike back behind the tightening lines. He threw the Scythe to Vi and waded through the girls to fight with his own sword. For a few moments, he couldn't understand why the girls were having such difficulty with the vampires. His own sword sliced through their lines like butter. Then a realization hit.
He cleared a bit of space and pointed to a girl. “You. Go get Willow, give her a sword and bring her back here. And get Xander too.”
With their lines so diminished, he and Vi focused on keeping most of the Turok-han off the girls and the girls lined up in a small semi-circle around the entrance, denying the vampires access to the world up above.
A minute later Xander and an almost insensate Willow arrived. Xander jumped immediately into the fray, laughing at how easy it was to kill the ubervamps now. Willow also perked up as soon as she was given a sword and pointed at the vampires. Giles and Vi continued their task, though it was now much easier with Xander and Willow thinning the crowd before it got to them. Almost no Turok-han got to the girls behind them.
“Oi!” Spike called from the ground behind them. “I think this thing is finally AARRGGHH!!” Spike cried out in pain as he touched his medallion.
Another minute of fighting and Spike's body shot a wave of magic through the roof of the cave. He started shooting sunlight at any nearby Turok-han, dusting them instantly and clearing the area around the stairs. “Rupert! Get them out!” Spike yelled.
“Xander! Willow!” Giles yelled. Then to Violet, “Vi, lead the way.”
The two young adults came racing back to the entrance at super speed. As the last of his charges cleared the stairs, Giles looked back to make sure they didn't leave any wounded behind.
“You've done your part. I've got the clean up.” Spike apparently mistook Giles' pause to double check as hesitation to leave him. It most certainly wasn't,
but whatever was happening was clearly very bad for the vampires and very painful to Spike, so he wasn't about to tell him. Giles gave him a quick nod. “Good show, old man.” He started up the steps to find a lot of blood at the top and Robin Wood waiting for him. “That's everyone.” Giles told him. Neither Buffy nor Faith were coming.
Robin swallowed but nodded. “We were attacked by Bringers and were too spread out so we fell back here. We lost Andrew and Dawn was seriously hurt. Violet tore through those bastards like paper when she came through.”
Giles just nodded and started moving. “Lets get out of here before the whole place comes down on us.”
“We were doing a lot better than we thought we would. They were coming in ones and twos. That let us gang up on them and take them out. When their numbers started increasing and we still didn't have our powers, I started pulling the girls back into a line instead of groups of three or four. I had half of them reformed when Buffy went down. Spike and Faith both noticed and got distracted. Spike was close enough to drag behind the line when he got wounded. We couldn't get to Buffy or Faith. We lost half of the girls when we lost our three heavy hitters. We knew Faith was gone when Vi got her power and none of the rest of us did. Then you arrived. What happened? Why didn't we get our powers like we were promised?” Rona demanded.
“I- I- I don't know.” Willow sobbed/moaned/cried. “I did the spell just like I was supposed to. But instead of activating the Slayer in each potential...”
“It activated the Slayer in each of us.” Giles finished. “The- the enjoining spell... we used to defeat Adam. A- a- a bit of the Slayer essence was left behind in each of us. If you remember the dreams we shared after that event. Apparently the spell activated the nearest Slayer essence, which would have been you, Willow. And your activation activated Xander and myself as we shared in that spell with you.”
“So, we're all Slayers now?” Xander looked around at the remaining members of the Scooby gang. It hurt to lose Buffy. He'd thought she'd been invincible. But now, at least, she was finally at peace. Plus, he had shiny, new Slayer powers to try out.
“Well, it's not thousands. But four Slayers is two more than we had before. And from what you guys said, you were doing pretty okay with just one.” Violet added.
“We're doing 'pretty okay'? Is that what you want to tell the 21 girls we left behind in that cave?” Rona asked.
“It's what we have to work with.” Giles said calmly. “Friends died but the world is still here. Such is the life of a Slayer or a watcher.”
“Or both.” Xander slapped Giles on the back.
A/N: Read a story where Xander kept a piece of the Slayer with him along with Hyena and Soldier. This was the result of my musings on that. Don't really have anywhere to go with this. Just pointing out that while dramatic, waiting until they were in the cave was a stupid move. This is actually fairly mild compared to worst case scenarios that could have happened if the spell didn't go exactly right at exactly the right time.