The Princess Bride is a short Pictish storyline quest which is started after you finish A Gaelic Alliance. After talking to Caustantin mac Fergusa, you should talk with Fingualla ingen Donncoirce about the wedding and ask what your role is in the ceremony. When you select the option: Tell me what I am supposed to do., the game will fastforward to the next morning when the ceremony begins. Before the two are officially married, Grianne storms in the room with news that the village is being attacked by vikings, under command of Skakki. He is standing in the middle of the town with his crew waiting for you.
Final showdown with Skakki
Before this battle begins, you can exchange some words with Skakki.
The battle can be quite difficult, because Skakki has high DR and can deal a lot of damage with his dane-axe Lawless Witch combined with the swipe ability. In case he comes into melee range, a swordman with the heavy swing ability can bypass his high resistance to physical damage. Else the archers have to shower him with arrows. Also it is recommended that you have two or three archers and some shieldwielders (including a tank) if you take a defensive position near the stairs until Skakki is incapacitated or killed. Using barricades and fire pots wisely can also turn the battle in your favour.
If you kept Skakki alive, you can talk to him before you personally kill him, or you can spare him and let the King judge him criminal acts. When you return to the kings hall victorious, you will receive your reward.