The Assassinated Man
Here's an odd one for you: you're to assassinate someone, but don't kill them. Francois Motierre in Chorrol owes a lot of money to the wrong kind of people and can't pay. An enforcer has been hired to teach him a permanent lesson and your job is to fake his death, in the presence of the enforcer, who will then go back to the wrong kind of people and report that Francois is out of the picture. To help you accomplish this, Valtieri furnishes you with the "Languorwine Blade", which is coated with a special poison (for which he also provides the antidote) that will make it appear that Francois is dead.
If you pursue the conversation topics with Valtieri, you'll learn that Motierre had to make special arrangements with the Black Hand to do this fake assassination. Since Sithis demands blood for every contract, the special arrangements were that Lucien Lachance got to kill Motierre's mother. Nice fellow, huh?
So hop over to Chorrol; the north gate is the shortest route. Motierre's house is on the corner, next door to the house with the "For Sale" sign in front of it (your house, if you've already purchased it). You'll have to pick the lock to get in and Motierre is right there, waiting for you. He'll explain that the enforcer, an Argonian named Hides-His-Heart, is already on the way. You're to fake his death, make sure that Hides-His-Heart sees it and lives to report it to his employers. So make sure that your Languorwine Blade is equipped (it's possible to fail this quest by actually killing Motierre), listen to the exchange betwen Motierre and Hides-His-Heart, and stab Motierre (once) at the appropriate point.
Now comes the hard part: keeping Hides-His-Heart alive to report back. He will start beating on you as soon as you do the deed on Motierre and will chase you when you leave the house (you weren't planning on just standing there, were you?). Now if you hadn't noticed, the town guards really take offense when people start chasing other people with swords in their hands. Even if you don't lay a finger on Hides-His-Heart, the guards will (yet another opportunity to fail the quest). So as soon as you leave the house, beat feet for the north gate. Once you leave town with Hides-His-Heart still alive, that part of the quest is done and now you're left with reviving Motierre.
You'll need to wait a day, so find something to occupy yourself. There are lots of ruins and caves to explore, Oblivion Gates that need closing, Miscellaneous Quests in the Chorrol area, or just find a quiet place and wait for 24 hours. When the time is up, you'll find Motierre all laid out nice and neat in the Chapel Undercroft (which isn't even locked for this quest). Activate him (make sure you have the antidote in your inventory) to get a journal entry completing that part of the quest. There only remains the problem of getting him to the Gray Mare for his getaway.
Oops. Seems he forgot to tell you about the part where reviving him constitutes desecration of the family crypts and how the angry spirits of his ancestors will want to punish the desecrator. Three or four leveled zombies will appear in the Undercroft, but there should only be a couple of them between you and the entrance. Keep Motierre alive and escort him to the Gray Mare. Once you're out of the Undercroft, you shouldn't have anything else trying to beat on you (unless you've got a bounty problem with the city guards or perhaps the odd Mythic Dawn assassin).
Once Motierre is at the Gray Mare, the mission will be complete and you can report back to Valtiere for your payment, Cruelty's Heart (a leveled Fortify Strength and Willpower enchantment) and a promotion to Eliminator. This is also your last contract for Valtiere, so you'll receive a key to the Sanctuary (which lets you enter and leave through the well) and he'll offer to turn you into a Vampire, if you want. If you'd rather pass on it for now, the offer remains open until the "Purification" quest from Lucien Lachance. Just use the "Dark Gift" conversation thread if you'd like to take him up on the offer later.
You will not receive anything if either Motierre or Hides-His-Heart died during the mission, but you can get the the promotion and key after completing the first contract for Ocheeva.