How can I get the Sanguine Rose that summons a Dremora?
This quest will only start once you reach level 14. You will need to meet with Sam Guevenne to start the quest. He will randomly spawn in any of the inns or taverns across Skyrim, including those in towns, not just the ones in hold capitals.
Source and more info at: http://www.uesp.net/wiki/Skyrim:A_Night_To_Remember
You can meet a man at level 15 to 20 in Whiterun named Sanguine, and he challenges you to a drinking competition. One problem is that you need to down three drinks in order to win the contest, and that's where the real fun beings.
He will spawn at a random bar across skyrim.. When you find him, he will challenge you to a drinking competition.. Then you undergo few tasks and then meet him at a fort.. Then when everything is over, you will get the weapon..
If you go to any of the taverns in Skyrim, then once you are at least level 14, he will challenge you to a drinking contest and give you 3 flagons. After you drink them, you will blackout and wake up to a woman, who will say that you had blacked out and then send you on a quest. You will get sanguines rose as a reward.