The quest Kingslayer begins after finishing Punching Through. Travel to Eoforwic (York) to join the siege on the city. There are several openings to enter the city and get to the King of Northumbria. Fight your way to the King's Hall. If Aethelred is king there are one of two options:
- Attack the king and his guards.
- Convince Wada to side with you and skip the battle (requires Diplomacy 5 and High Sense). This will however result in the death of the king.
After disposing or sparing the king, talk to Caustantin mac Fergusa to complete this quest.