Whether they were introduced in the Season Premiere or not, it's time for the season's Big Bad to make a play and get a bit of spotlight time.
This could be a scheme that looks like a MOTW and can be stopped like one but the overall threat proves larger than expected, a feint where the Big Bad is introduced under false pretences, even a blatant attack that leaves the Cast wondering what just happened.
So this seems like a good place to discuss what makes a good Bad.
There are many (often contradictory) ideas on villain construction (here's an entire thread of them) so I'll focus on what works for me.
Face time where you can't kill them: See the Mayor being immortal, Spike chatting with Buffy at a safe distance, Angelus in particular and his head games. This is a character, not just a monster. They could be relatable as well, if you can make it work.
Power: The average Big Bad is more powerful than any MOTW, more powerful than any of the Cast on their own, and more powerful than last year's Big Bad. The show started with a pretty old vampire and ended with Evil Incarnate. This can vary, but undercutting the threat can leave the players unimpressed.
Betrayal: A villain that tricks you hurts more than one that makes it clear you're not going to be best buds from the outset. This also increases their face time, and lets the Cast see...
Weaknesses: Other than hitting them a lot. Although hitting them a lot should be very satisfying by the time it comes down to it.
Metaphor: Not essential, but a definite added bonus. Angelus as the boyfriend who turns scummy after sex, Faith as the temptation of power.
Neat gimmicks: These aren't essential, but if you need some, try here.
Bigness: Fake Big Bads feature in several seasons before the real big brewin' evil shows up, and get killed off (sometimes by the actual Big Bad) after a couple episodes and a decent fight around the mid-season break.
The most successful Big Bad I've created is Victoria Valdermar in The Watch House. She's just a vampire, but above that...
She has screentime outside of fights. As noted above, she does the Bond villain bit of showing up to chat when the PCs are in no position to attack her.
She's similar to the group leader, Milli, in some ways, and very different to her in others, so a very unfriendly rivalry has developed. And she will insist on flirting with her boyfriend...
Above all else, I've tried to keep her consistent while showing different sides of her in different situations. She demonstrated a further depth of evil in season four, but it wasn't a real surprise because we knew she was capable of it from the get-go. A consistent villain gives the players something to hate that they can rely on in a pinch.
And she's been around long enough that the PCs have differing views of her. Milli, our hero, despises her immensely, partially because Jake was always scared but (never admitting this) intrigued by her interest.
Example: Siren Song
A routine patrol goes sideways when the vampire being chased disappears without a trace in a dead-end alley. Where did he go, and... is that the time?