Xander Harris in DC Universe

Imagine a book, next to a whole bunch of other books. Each book is an individual dimension, and each page in that book, is an individual reality. These books can grow or shrink as the realities in the dimension expand, or collapse. Occasionally aspects of one dimension can be picked up by beings in another dimension, which can then affect their realities. Usually these aspects appear as stories or legends, though occasionally there are people who can walk between these realities, and even more rarely these dimensions.

Buffy Universe:

Sunnydale, California May 2003

Willow starts to chant, calling on her Goddess. They scythe glowing white in her hands. You hear Buffy's voice talking reverberating through time and space.

"From now on, every girl in the world who might be a slayer... will be a slayer. Every girl who could have the power... will have the power... can stand up, will stand up. Slayers... every one of us. Make your choice. Are you ready to be strong?

Casting the spell, white light spilling from her. Willow's hair is completely white, and she's still glowing with white light from the spell. Willow's facial expression is one of divine ecstasy as she looks heaven-ward. Willow pants as the power runs through her. When the scythe stops glowing, so does Willow.

The spell spreads out, covering the Earth and keeps going, through the realities, awakening Slayers calling them out, through dimensions, finding the mystical marker in young girls, and waking them up.