Still, there is a questions remains: HOW? How she just possessed you? If she can so easy leave Hall of Echoes, why make all that complicated mess, there was much easier ways to do the things.
Ok, let's clarify further.
How? Well, she doesn't really possess you, now does she? You get dragon powers from Talana and you make your way through the game with the powers you have, but Ygerna just steers things. She doesn't actively control you, she just suggests things you need to do in order to – you think – kill her, but in fact – as she well knows – to free her.
She can't, after all, leave the Hall of Echoes, but she talks to you from within it. You never actually see Ygerna until you meet her in her memory. This means that you only actually meet her real self at the very end of Flames of Vengeance.
You see a dying Talana; you see a dead Talana on the ground in dragon form, but you never saw Ygerna. She is just the impostor in your head whom you have no reason to doubt as even Zandalor never guesses her scheme.
I dunno about Ygerna, it's just random make-up for Div2, but, if we accept the whole Divinity Universe (DD+BD+story_before_DD), then the Divine One was correct: Damian is the Lord of Chaos and he can't be killed/destroyed. he always come back. So far I haven't found any sane reason why Larian ruined the previous setting and changed almost everything in Div2 series.
You can dislike the story line of course, but it does not ruin the universe. The story of Divinity II is just one of the many ways you could envision the story of Damian and the Divine continuing – keeping in mind the real life constraints there are when making a big game with a relatively small team.
One thing though, I must correct, and that is when you say 'Damian is the Lord of Chaos and he can't be killed/destroyed'. This is not quite true. Damian has the essence of the Chaos Lord in him, but he isn't the Lord of Chaos yet! This is a very important thing to keep in mind when thinking of the way the story develops and will develop. Again, an in-game character explains this in more detail. This time it's Thaddeus. Among the many things he says, he tells you the following about Damian:
Player: So Behrlihn is a follower of Damian, then.
Thaddeus: Yes, in so far that the human Damian is now the shell in which the Chaos Demon dwells and evolves. (...)
Player: If I understand correctly, Damian is a budding Demon – one who has not yet reached full growth.
Thaddeus: Oh, yes! The true Lord of Chaos has not yet been freed: the Black Ring was never as strong as the wizards that once supported him. But they did succeed in kindling his spirit within that creature Damian, where as yet it is but a flame waiting for conflagration. I don't know how that sinister design could be brought to reality, but never doubt he and his believers work tirelessly to bring the Demon about!You can always go back to him in the game and ask him about all this again. He doesn't mind, I assure you!
