With the Amulet delivered, your next task is to find Martin Septim, the last heir to the throne, in Kvatch. Kvatch is located west of the Imperial City. As you approach the city, you'll witness a horrific scene: an Oblivion Gate has opened, and Daedra are pouring out, laying waste to the city. This is your first encounter with a full-scale Daedric invasion. Speak to Savlian Matius, the captain of the Kvatch Guard, who is desperately trying to hold the line outside the city gates.
Savlian will ask for your help in closing the Oblivion Gate and rescuing any survivors. This involves entering the Oblivion Gate itself, a terrifying dimension filled with Daedra and treacherous landscapes. Navigate through the Deadlands, defeat the Daedric forces, and reach the Sigil Keep. Your objective inside the Sigil Keep is to retrieve the Sigil Stone, which powers the Oblivion Gate. Once you acquire the Sigil Stone, the gate will collapse, and you'll be transported back to Cyrodiil. After closing the gate, you'll need to help Savlian Matius retake Kvatch before you can finally meet Martin Septim in the Chapel of Akatosh.