The Fall of Pictavia is a Northumbrian or Northmen storyline quest which can be started after Into the Dark. Wada asks the player to kill Caustantin mac Fergusa, King of the Picts in Perth.
Travel from the Mountains to Perth. In Perth, the first fight immediately starts with Picts Scouts. This fight is considered easy, as the enemies are on level 4 and 2. After the fight, head towards the bridge for the second fight, which is considerable more difficult. Enemies are on level 4 and 2 too, but is there many of archers. In case you recruited Dun Phris during Fort of the Thicket, you will receive reinforcements from Aiblinn and some other villagers.
Third fight with guards in front of the town hall is hard. You must separate two groups enemies with traps or fire. After third fight, you enter to Caustantins hall.
In Caustantin's hall is the last fight with Caustantin and his bodyguards, which are located near the thrones. He will receive reinforcements in the third turn. After that you meet with Aethelred and complete the quest.