King Alistair is a side quest in Dragon Age II. It is received after the completion of On the Loose via a letter to Hawke found on the writing desk at the estate.
This quest is available if you select a pre-made background within Dragon Age II in which Alistair is king. This quest also appears when starting a Dragon Age II game with an imported Dragon Age: Origins game in which Alistair was made king.
Walkthrough
The letter at the writing desk appears after completion of On the Loose. After receiving the quest, head to the Viscount's Keep. A cut scene will play with Knight Commander Meredith berating Alistair over three mages who had escaped to Ferelden. After Hawke is introduced to Alistair, Aveline, if she's in the party, will kneel before Alistair, provoking a wry remark from him. Anders will also have lines here if present, Fenris and Varric have a couple of lines as will Isabela. If the imported save file is a Dalish Warden and if the boon given at the ending was land for the Dalish, Merrill will have some lines as well; if the imported save file isn't from the Dalish Warden, Merrill will still have a line concerning the Circle of Magi. Alistair will then discuss a threat to Ferelden from Orlais, and will tell Hawke to keep Kirkwall safe. If Hawke asks what the greatest threat facing Kirkwell is, Alistair will say wryly that he'd say it's Meredith.
Result
Completing the dialogue will complete the quest and a codex entry for Alistair will be added.
When Alistair asks you how to fix the city, replying with "I could be viscount" will result in 
Varric: friendship (+10)
Trivia
- Alistair says "Swooping is bad" when he first meets Morrigan in Dragon Age: Origins.
- If you import from a save where Alistair and the Warden had a threesome with Isabela, she will have an amusing dialogue. If he didn't, he will simply state that she looks different, probably refering to her character design, to what she answers "Don't we all?".
- If you have Anders in your party, he will ask if Alistair was a Grey Warden, he'll reply "That's the rumor" and then turn the same question to Anders, which he replies "That's the rumor". Alistair finishes with "Well, we certainly get around".