BtVS by Whedon and Mutant Enemy. Story starts somewhere around Season Four's This Year's Girl.
Los Angeles, CA - February, 2000
Faith the Vampire Slayer struggled for consciousness, fighting off the last vestiges of the coma she'd been in for months. When she opened her eyes, she saw a hand reaching for her. If she had been a bit more coordinated then she'd have snapped the arm in two, but her own limbs felt unusually heavy and the couple seconds it took to move them into position gave her target enough time to withdraw.
"Easy, my girl," a familiar voice said. "We don't want to hurt ourselves on the first day back."
Faith turned and saw her father-figure, the dark sorcerer Richard Wilkins the Third, standing by her bedside. He had a warm expression on his face and he was still apparently human.
Faith reached up, muscles aching, and ran her hand over the line of his jaw. "It's you. I thought you'd have Ascended into a form you'd be more comfortable with by now. Not that I'm complaining. Unless..."
"It's true, Faith. My plans to become a giant snake have failed to come to fruition. The 'good' Slayer and her team brought that much off, though they never had the chance to enjoy it. If I hadn't still been under the influence of the hundred days of immortality I'd be as dead as they are now. Even so, it was a long, tough climb out."
Sighing, Wilkins rummaged through the dresser by Faith's hospital bed. Selecting a folder, he sat it on Faith's chest. As he helped her move into a better position, photos spilled out over the sheets. "These are some aerial shots of the disaster. Apparently they decided that, as my power source was the Hellmouth, they'd do everything in their power to close it. It must have been that the fact Sunnydale would turn into a giant sinkhole was left out of the manual for whatever doohickey they used. I know that my becoming a Demon would have been a pretty big deal, but that's no excuse for the loss of life and the mess they caused."
Faith frowned, rummaging through her memories. It seemed she was forgetting something.
"We're staying nearby until I can raise enough money to finish my dig. You see, there are some artifacts that need to be recovered before we travel to the next Hellmouth, although I'm afraid your Playstation has almost certainly been hopelessly smashed."
"Dawn," Faith said, sitting bolt upright. "Where's Dawn?"
"Oh." Wilkins looked around, then stepped aside apologetically. "Your sister's right over there."
Faith smiled as Dawn Lehane's sleeping form came into focus, crossbow leaning against the side of the chair she was sitting in. As Faith spotted the weapon she'd given Dawn, her smile faltered slightly. She really hoped her sister had grown out of the habit of using kittens for target practice.
"She was by your bedside when the disaster happened and she's been guarding you almost every day since. It's taken a lot of effort to make her attend classes regularly, but I think I've gotten her to understand the importance of a good education." Wilkins reached into the dresser again and pulled out a bunch of brochures. "Speaking of which, now that you have a new identity and there aren't any pesky heroes to be worried about, let's talk about colleges for you."
Cleveland, OH - January, 2001
Glory, the Great and Powerful, grimaced as she fought the Slayer under the railroad bridge. It wasn't her choice of battlefields and she was afraid the scummy water would damage her high-heeled shoes.
The reason for the location shortly became apparent as a small troll dropped from the darkness above her to wrap its filthy arms around her shoulders. Screeching, she broke off from the battle to break the demon's neck.
The few seconds respite were all Faith needed to reach in her pocket and don what appeared to be an enchanted pair of brass knuckles. As the dead troll fell to the ground and Glory automatically began brushing dirt from her dress, Faith ran in close and grabbed her by the hand.
As the gleaming metal made contact with the Hellgod's flesh, the air around the two women began to vibrate. They began to vibrate as well, energy visibly traveling through their arms. A bright flash of blue light separated them, sending them slamming into bridge supports on opposite sides of the archway. The magical artifact clattered to the ground, its metal glowing red hot.
Wilkins emerged from the darkness, hands clapping. He tossed a pair of handcuffs into the air and Glory's body moved at superspeed to catch them. An eyeblink later and she was next to Faith's body, using the tool to restrain the groaning Slayer.
"So, daddy," Faith said, obviously enjoying the power rush that possessing Glory's body gave her. "What can we do with Slayer parts? I imagine they'll fetch a lot of money on the black market, especially if we sell them off an inch at a time."
"Wait." Glory tried resisting as she was pulled to her feet, but the Slayer's body she found herself in wasn't as nearly as strong as her old one. "You don't want my body. It's defective. It has a timeshare system and it's not properly designed for this world. If you don't get a steady diet of healthy brain energy, you'll lose your mind."
Wilkins approached them, walking past the body-swapping Draconian katra he'd originally intended Faith to use on the previous Slayer in the event of his death. He was getting a lot more enjoyment out of how things had actually turned out. "I think we can find some way around that, but if not, it's not that much of a problem. After all, I've seen the building you've started to raise in that abandoned construction site." He grinned. "You see, I'm running for Mayor of this town and I figure, if you can influence crazy people enough to do something as coordinated that, I can get them to flood the voting booths."