everyone must run fast 是一个完整的句子,
where 在这个句子中充当状语成分,是关系副词,修饰先行词game.
game是抽象名词,表示一种游戏,“这是一个每个人都必须迅速奔跑的游戏”
同game类似的有抽象概念的先行词还有 situation,stage,point,etc.
e.g.The accident has reached a point where both their parents are to be called in.