Disclaimer: I do not own Buffy or Star Trek. Joss Whedon and Mutant Enemy owns Buffy and Paramount owns Star Trek.
Q watched as the scooby gang, as they called themselves, their friends and other potentials walk to Sunnydale High School to confront the First Evil and its army. The Continuum had ordered him to Earth, in this dimension, to observe the result of that battle. The Continuum knew if the scooby gang failed, it would have dire consequences not only for their dimension, but every dimension in existence. They also knew their existence was at stake. The Q rarely came to this dimension. This place frightened them...though they would never admit it.
Q watched as Willow cast a spell over the scythe that made all the potential slayers in the world, slayers. Q saw the slayers open the hellmouth with their blood and watched as they battled the Turok-Han. He saw a few Turok-Han escape the hellmouth, who made their way to the upper level of Sunnydale High School, and fought others in the corridors.
Q saw Buffy wield the slayer scythe and wondered what type of power was in that strange weapon.
He was watching Spike when the necklace around his neck began to glow and suddenly, a bright white light, brighter than the sun itself, shot out from said necklace destroying Spike and all the remaining Turok-Han.
Q saw Buffy Ann Summers, the longest living slayer, depart the crumbling building and started jumping from rooftop to rooftop until she reached her destination and jump on top of a yellow school bus. He watched as they drove a few miles out of town when the bus stopped...and saw them get out of the bus, turn and watch as Sunnydale sunk into a crater. Closing that hellmouth forever.
However, Q being Q, knew another hellmouth had immediately opened elsewhere on this world and knew they would start the battle all over again – which will go on until the end-of-days.
Q snapped his fingers and left.