Skyrim:Guests for Dinner

Quick Walkthrough
# Travel to Bloodchill Cavern
# Speak to the waiter
# Sit at the table and read the note
# Defeat either the vampires or the guests and Dawnguard
# Speak to the survivor to receive leveled gold and the key to your new home

Detailed Walkthrough

Player as a Non-Vampire
When you arrive at the manor, a number of guests are milling around the front door listening to a bard. Just inside, another reveler is drinking and dancing. Proceed down the hallway to the waiter. No matter what you say, the outcome will be the same; with him handing you a dinner menu (Version 1) and directing you and the other guests to your seats.

Sit down at the table, in a place of honor across from the door. Once you sit down, read the note on your plate named Please Read Aloud to gain some insight into this mysterious dinner party and proceed with the quest. After you read the last page of the letter, the waiter will approach Galtius across from you and bite him from behind, killing him. As the other guests rise from their seats in a panic, the bard in the room, the waiter, and two gargoyles will attack. 

Deal with them, and be careful of the waiter: his lightning attacks are strong enough to one-shot lower-level players. Resist magic gear and followers might help. Once all the vampires and their stone minions are dead, speak with the "survivor." Some of the other guests may have survived your battle but the one to speak with is the Orc Durian, who is revealed to be a member of the Dawnguard:

:The vampires are all dead. And the will says the house belongs to me.
:"Very good. But you've created a debt between us. Here. Take this. We are even now."''

He will then hand you a leveled amount of gold and a key, and you will become the official owner of the manor. This completes the quest. The doors along the entry hallway will open and you can begin using your new base.

If there are any surviving guests besides Durian, they will return to sit at the dinner table amidst the carnage. Once you leave and reenter the cave, all of the bodies and debris will be cleaned up (including the vampiric bard near the entrance, which was presumably dispatched by Durian on your way into dinner), any surviving guests will have left, and the dinner table will be cleared out of the trophy room to allow your unimpeded access.

Player as a Vampire
If the player is a vampire, things play out a little differently. When you arrive at the manor, pass through the guests and speak to the waiter at the end of the hall. After choosing one of four options, the waiter will give a generic response and hand you the dinner menu (Version 2). He will then turn and lead you to the banquet table.

Once there, sit and read through the feast note Please Read Aloud. After doing so you are instructed to begin the feast, by drinking the blood of the already enthralled Parven. At that point, the waiter will approach Galtius and bite him from behind, killing him. After Galtius's death Parven will also terminate and the Dawnguard will enter the building, initiating battle.
 
After disposing of the Dawnguard and the remaining guests, speak to the waiter once more.

:The Dawnguard and the nobles are dead.
:It would seem so.
:The note seemed to imply the last survivor would get the house.
:''I'm so glad you succeeded. Here. Please take this for your trouble.''

At this point the waiter will give you the key to the manor and some leveled gold, completing the quest. Three more dialogue options are given; all will end the conversation. The doors along the entry hallway are now open for use, and when you leave and reenter the cavern all of the destruction from your fight will be cleared away.

Notes
After the attack in the non-vampire version, Durian is renamed Dawnguard even if the player has not started the Dawnguard quest line.

Quest Stages