Question about Scaith and the Void (SPOILER)

Don't know if I should add spoiler tag here in Lore section or not. But, well, just in case.

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I've been wondering, why does Scaith become our enemy?

I thought as Caith Sith (her familiar?) was on our side, she should be our ally.
But she ended up fought against us in Dun Scaith. Why?

And what is the Void? From my understanding, it's where Diablos came from.
But what exactly is it? Is the Void created the moment balance between Light and Darkness crumble, like what happened to the First World where Warriors of Darkness came from?

Cait Sith is the familiar of High Voidmage Cessair not Queen Scathach.

The Void is the 13th Shard which fell to Darkness. Voids are created when light or darkness become completely dominate on a shard (the first is headed towards becoming a void of light but Minfilia was sent to try to save it).