Ooh, a potentially awkward one, that. There'd be a lot of destruction, but a good half of it would be a sudden and dramatic increase in Forsworn activity along the road from Solitude to Markarth and possibly a hastily organised riot in Markarth itself to make sure Igmund doesn't turn up. That dealt with, the real danger in that quest is Madanach having too good a time at the party to actually do the quest, what with chatting up Elisif and offering to get that Wolfskull Cave business sorted out for her, and slipping Skooma into Erikur's drink and watching the fun unfurl.