Does Brynhildr stop my opponent from casting Edge of Night if it was hidden?

Yes, Brynhir Thundersong prevents your opponent from playing Edge of Night from its hidden state.

According to the ruling for Brynhir Thundersong:

• "Opponents cannot play cards on whatever turn Brynhir is played."
• "Cards from hidden are also played, so Brynhir's effect prevents revealing them."

Because playing a card from its hidden (facedown) state is considered "playing a card," it is restricted by Brynhir's "When You Play Me" effect once that effect has resolved.

Important Nuance: If your opponent has already played Edge of Night and it is currently pending on the chain, it will still resolve. However, if they have not yet initiated the action of playing it from its hidden state, Brynhir's effect will prevent them from doing so for the remainder of that turn.