Lore question: Why the Dragon Soul?

A long time ago I read the books where the Dragon Soul (later called the Demon soul) was used by Deathwing to fight and subjugate the other dragon aspects, the puny elves, and the hordes of the Burning Legion.Then after much heroic effort a human destroyed it using a scale from Blackwing.

What I remember was that Deathwing was immune to the dragon/demon-soul’s abilities, as he chose to not add his own power to it when it was forged, as he wanted to have the talisman for nefarious deeds. This is why it was his scale that was able to destroy the talisman.

Why then would Deathwing be vulnerable to the Dragon Soul now? I don’t get it.