The Couslands are forced to deal with betrayal by Arl Howe in the quest Howe Treachery. You went to sleep after the pleasant family reunion, only to find that Arl Howe's men had infiltrated the castle, and had begun killing everyone in it. Your mother doesn't know what happened; she has joined you as you search the halls of your home for the rest of your family.
You awaken in the middle of the night to Dog barking. Equip your gear immediately and kill the two attackers that invade your room. Exit your room and take out the two Howe soldiers at your father's bedroom door. Once they are dead, your mother joins your party after a conversation. Make sure to loot the chests in your father's room to the south. Open Fergus' door to find Oriana's and Oren's bodies.
Open the door to the north to confront four enemies, including a Howe archer. After that skirmish, proceed north. During the next conversation with your mother, choose "What should we do?" followed by "Is there nothing else we can do?" to receive the side quest The Cousland Treasury.
Head east and try persuade the servant to stand and fight with you. You will fight several groups of enemies, possibly without a break between. Make sure to check side rooms and rubble for loot and get the items from the treasury. Be aware of an attack after looting the Family Chest.
Enter the main hall prepared to join in a battle where Ser Gilmore and two guards are fighting off 10 enemies, including a Howe Mage. After the fight, Ser Gilmore tells you that your father was heading towards the kitchen pantry.
Note: If you speak to Ser Gilmore before he activates the dialogue with you, you will not receive the key needed to open the door.
Prepare yourself and exit the main hall from the northern door. A guard is fighting off two enemies outside. Recover and then head west. The next group of enemies includes a Howe Knight, a more powerful "ranked" enemy. Use caution.
You are now free to enter the kitchen and open the pantry door, which will initiate a conversation that ends this quest. Be sure you have finished exploring the castle and completed the treasury side quest before opening the door.
You receive XP (~1280 at level 2)