Normally game ends then, you see the ending movie after you kill Janus. Did you download the hotfixes from the main Divine Divinity site? You can find them in the support section of their site.

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Replying to:

I've been having problems at the end of the game. I've recently killed the 5 Black Ring members and the Demon of Lies, but how do I do away with the kid on the altar? (There's also some bug that froze the death animation of the Demon)