Void Dragon is the final boss of Divinity: Original Sin.
He is the one who contaminated the Source, and had been sealed in the Godbox. But Astarte, allured by the Trife, mistakenly opened the Godbox, and the Void Dragon was set free. Until now, Astarte has been fighting against the power of the Void, but the Void was growing time after time, meaning that the Void Dragon will once again threaten Rivellon. And finally, Leandra who has been corrupted by the power of the Void summons Void Dragon at the final battle in the First Garden. Our two Source Hunters awakened as the Guardians go in to the First Garden and fight against him once again.
He has extremely high chance to dodge attacks, so Astarte's Mass Bless spell comes in handy. If Astarte falls, you automatically become game over, so you must keep Astarte in safe place, using Feather Drop spell. Note though that despite low Tenebrium resistance Tenebrium damage will actually heal him so it's not advised to use Tenebrium enchanted weapons.