It's already been explained. She made a pact with an entity from the Shadowlands. When that entity becomes stronger, she becomes stronger. That's why she waged war against the Alliance - to feed that entity souls from both sides. That's also why she helped Azshara free N'zoth. Nearing the end of BfA, she finally became strong enough to do what she did in the cinematic.
To my understanding, the Helm of Domination was a conduit between the physical realm and the Shadowlands. Once it was broken, the Shadowlands started to seep in through the leftover wound.