When you're ready to convince her, here's the correct Mythal answers in Dragon Age: The Veilguard: Solas is helping me > Forgotten him. We must stop the Blight > You stood against it. We must stop the Gods > Put the Blight aside?

The Veilguard and uncovered all the pieces of Solas’ backstory, a new quest will unlock that gives you a brand new power. Go to the Crossroads and you’ll find Morrigan who explains that you’re about to encounter a vestige of Mythal,

One of the requirements to achieve the secret ending in Dragon Age The Veilguard is to obtain Mythal's power. To achieve this, the party must either battle with or come to an understanding with Mythal.