I saw this and got really excited before I remembered I've already owned the game for a year. 
I think Madrar got it, she was asking for advice.
All signs point to Ireland 1066, is all I'm saying.
As for Sebastian.
Ugh.
Just.
Ugh.
I've admittedly only seen videos of his scenes.
But the man invades and tries to annex Kirkwall for 'sheltering Anders'.
The thing is, Anders is one of the few members of the Mage Underground left standing by Act III. The main people sheltering him at that point are the poor people the Chantry couldn't help but were helped by Anders, and Hawke and company. Including Sebastian. 
And the rest of Kirkwall is distinctly not cool with Anders after he blows up the Chantry. Orsino is horrified. Meredith is willing to wipe out the Circle over it, even though they had nothing to do with it. If Anders is alive, Sebastian's beef is with Anders and Hawke for sparing him. If he felt so strongly about it, he should have killed him himself right then and there. (Admittedly possibly a death sentence for himself, but then again, trying to take over and instill order in Kirkwall is a death sentence.)
So he invades a city-state to find a terrorist who isn't there to avenge the death of a pacifist woman on people who hate the terrorist as much as he does. And then gets knocked out of the city by an irate Aveline and the support of some Inquisition agents.
I do like some of his banters, it's just...ugh. I can't describe the hatred I have for being people misblamed. Like the old, "If that one student doesn't stop screwing around, I'm punishing the whole class," It never made me try to stop the student, it made me despise the teacher for resorting to sloppy techniques and being that lazy. The two main results are either that the students turn on that one, or they unite against you. Both results are bad. It used to get me so furious that I could barely breathe. Actual serious instances of it are much worse. 
I apologize for ranting.
And my apologies to Sebastian fans.
POOR SEBASTIAN! </3
Ok, he didn't sweep me off my feet but after getting to know him I do kind of feel for the guy. Before his DLC, despite playing mages I was determined not to take sides between mages and templars - which left me feeling like there was a side to the story I wanted to be a part of but was missing from the game. For example, when it came to deciding how to complete a quest, my reasoning depended on the nature of the actions of those involved. Templars exercising personal grudges against mages = no. Same for mages against templars = no. Templars abusing power = not so good. Mages abusing blood magic = not so good also. Of course, I was inevitably forced to choose at the final battle (I assumed for future narrative purposes, which turned out to be true).
Sebastian came along and suddenly we had the side to the story that applied to me. Yes, he put many people off with his holier-than-thou attitude, but he didn't take a clear side against mages of templars. Rather, his approval depending on whether their actions were constructive or destructive.
He was also particularly found of Grand Cleric Elthina (I think that was her name?) and she of him. Sebastian's personal story involved him trying to work out what path he should take in life - to complete his vows with the Chantry, or step up to Prince of Starkhaven. Conversations between Elthina - and Hawke - were pivotal. He also attempts to persuade Elthina to involve herself in the dispute between mages and templars, and is frustrated that Elthina and the Chantry refuse to facilitate peace where (he feels) they could.
From this perspective - seeing Elthina die in the Chantry, my heart went out to him. Especially the cutscene following where he drops to his knees in agony! UGHHHH
And then he asks Hawke to kill Anders in return. I imagine Sebastian asked and did not kill Anders himself because... Hawke is badass and would have kicked his ass if she didn't approve.
Luckily, me and my mage Hawke sympathised with Anders cause, but 100% did not approve of his methods. Knowing Anders shared a body with a spirit which was capable of this? There was no telling how many other innocent lives were at stake if we let him live. I never had any problem with killing Anders, and playing through with a friendmanced Sebastian made it feel even more justified.
Unfortunately I am pretty sure Anders always intended to become the "martyr" for his cause, with all the implications of that legacy, and part of me is annoyed that I allow him that.
Sebastian, if rivalmanced is a Prince on an angry revenge mission if Hawke did not kill Anders (I supposed this was yes, punishing a city which learned to revere the blasphemous Hawke - but since I did not rivalmance him, that did not concern me). If friendmanced, Sebastian joins the Chantry - is merely an influential Chantry advisor to Starkhaven's court (albeit still a member of the royal family) - and encourages the invasion of Kirkwall on the grounds of dealing with moral corruption at its most foul (let's face it, during DA:I Kirkwall is still covered in Red Lyrium amongst other things...).Either way I don't think it is as simple as "punishing the class for the actions of one person". I do think that Sebastian sees Kirkwall as a completely lost shithole of a cause, in need of regrouping but unable to do so without aid (welcome or not).
In my DA:I game however, since I kill Anders and friendmance Sebastian, Kirkwall is not invaded, Starkhaven allies with the Inquisition, and Sebastian leaves to follow Hawke to Weisshaupt after the events at Adamant. 