The Scoobies have discovered a prophecy about Xander…

The 3rd generation Hellmouth son, conceived and born on top of the very mouth of Hell will search for his 2 mates, Death's undead yo-yos.

Death's undead yo-yos are two warriors who have experience the pleasure and pains of Hell itself and have returned from it stronger.

Once the Hellmouth son and Death's undead yo-yos are joined they will have the power to control the Hellmouth and call of this world's gates whether they be to the stars or to Hell itself. They will have the power to bring to this world peace for as long as they desire to live or watch it burn to ash.

Buffy immediately says that she's on of the Death's undead yo-yo but Dawn points out that Buffy is more like Death's paddle ball, Death gets a couple goods hit in and has to start again while a yo-yo you can go for hours.

So now the Scoobies start the search for Death's undead yo-yos

~*~

Death's undead yo-yos are Colonel Jack O'Neill (Stargate SG-1) and Dean Winchester (Supernatural)

 Jack died a number of times at Ba'al hand before he could escape and Dean died at the Trickster's hand an equal number of time.

I figured that Xander's family could have been in Sunnydale a long time perhaps a family of chaos mages on his father side. In hope of gaining more power he has sex on top of the Hellmouth but the plan backfire and he loses all of this magic when Xander is conceived and that's why he descends in the bottle