Lost City of Amdapor

[Velox]

Actually, I thought it was the WHITE Mages who dabbled in their aetherial power too far and ended up bringing over Diabolos and the voidkin

Diabolos was summoned from the void thru black magic (blm quest line explains the link).

However, what you are talking about is also a legitimate concern with both black and white magic. BLM and WHM utilize the aether of the world and living things to do their spells. In essence, they have to drain aether from other sources, which funny enough, is exactly what primal do to survive. That is why WHM and BLM have to keep their power in check, as over use of it saps the life out of the planet and they become no different than a primal.