(DM rolls 1d20: 11!) The horn costs you 50 GP. It looks nice, but you have no idea how close it is to the original. You peek into the Blue Iris's main room at 2 in the morning. Despite his dawn appointment, Sir Alec is still in there, throwing dice with the Hon. Percy and two nobly-dressed women. Timmins whispers, "Wait for him to go to sleep, or go in now?"
"We have a few more hours before dawn. Let's scout around for an entrance and exit point and when they retire I'll deepen their sleep with a spell."
"It's a normal tavern. Bar on the first floor. It's usually riotous with well-born guardsmen until about three in the morning. Private rooms on the second floor. I'm sorry I can't draw you a map; I've never studied it with an eye to burglary. I'll leave that to you. See you at dawn." He salutes you and heads towards the nearest path.
Hm... First, collect the forgery. Then to the Blue Iris. Perhaps we can use the liquid light as a distraction?
#1 and let's bring an ally for backup, Timmins might do nicely.