Read what Zandalor and Talana said. Damian messed up with Soulforge spell and "inverted" it. Something went wrong but even Zandalor had no clue what happened. So, his thoughts that Ygerna's resurrection will kill Damian can be wrong or correct as well. And, as you can see - he was wrong. Ygerna's ressurection - Damian immortality/invulnerability (Lucian says so @ the end)
Errors in the plot:
1. Damian's soul - the Lord of Chaos' soul. As we knew from DD1, only his material body can be killed, but no the soul. So, there is a question: how Soulforge spell will affect him? 2. What the point of immortality if Damian already is?
