The Assassined Man
The next mission is to fake the Heath of Fancois Motierre. Go to Chorrol and break into his house. Talk to Francois and arm yourself with Languorwine Blade (you just got it from Vicente). Wait for the Argonian Hides-His-Heart to appear. After a short scene, attack Francois with a blade and run for it. You can't kill the Argonian, if you want to receive the bonus from the Brotherhood. Leave the house and get out of Chorrol.
Wait for 24 hours, to give the townsfolk the time to move Francois' body to the Chapel. Go to the Chapel Undercroft and use the antidote on him. You'll be attacked by a few zombies. Get Francois out of there, and escort him to The Gray Mare. Talk to him once again and go back to Cheydinhal.
Vicente will once more advance you in rank and give you the well key (thanks to it you won't have to enter the Sanctuary through that annoying cellar any more, but directly inside through the well next to the abandoned house). He'll also offer to pass his vampirism onto you. The choice is yours.
The Renegade Shadowscale
This mission is not in the Dark Brotherhood's main course of events, but to get it you have to be its member. After doing some initial quests for Vicente, talk to Teinaava, who'll ask you for a favor. You are to find and kill a traitor named Scar-Tail, who's hiding in Bogwater, far to the south.
When you reach his hideout, he'll offer you a deal: spare his life and get his treasure (200 Gold hidden in a Hollowed-Out Rock a few steps to the East from the tent). You also get another Argonian's heart to ensure Teinaava you did the task (take it from dead Argonian laying under the tree to the west).
Teinaava will believe you no matter if you really killed Scar-Tail or made a deal with him. You'll get Boots of Bloody Bounding for this one.
The Lonely Wanderer
The first mission given to you directly by Ocheeva is to find and kill an elf named Faelian. Go to Imperial City Talos Plaza, to Tiber Septim Hotel. In there you'll find an elf Ataena, who'll tell you about your victim's habit: he's hanging out in Lorkmir's House in Elven Gardens most of the day. Now you have three ways to deal with the problem: break inside (Hard lock), or wait for him to come out. Then you can either steal the key from him or talk to him and bring him some of the drug called Skooma.
You can buy Skooma from a trader named Nordinor, who's running an illegal business in Bruma overnight. Grateful Faelian will invite you to Lorkmir's House for a party. In there, you can get the world rid of him without any witnesses.
Enter the Eliminator
Ocheeva wants you to go to Fort Sutch and switch Roderick's Medicine with the poison she provides. Do it without being seen, or you'll loose the bonus. Next to the place where you enter the Fort is a gate (Hard lock). If you're able to open it, you're one step ahead. If not, there's some sneaking ahead of you.
Sneak past the first guard so you can use the lever to the left of the door. This will open the passage. It's dark in here, the Nightvision spell comes in very handy. Sneak to the green map marker, and once you reach the closet, take Roderick's Medicine and put in Roderick's Poison.
You can leave the dungeon the same way you came in (a lot of sneaking included) or use the tunnels in the right part of the map (you might have to steal a key from one of the guards, if you can't manage to open all the locks). Go back to Ocheeva. If you managed to leave discretely, you'll get a cool looking finery.
Now that's what makes a good RPG. The idea is simple: you're one of the guests invited to a competition taking place in Summitmist Manor in Skingrad. Everyone else think that the goal is to find a chest filled with gold. You are the only one who knows that nobody is leaving the manor alive - except you.
Your mission is to kill all 5 guests without anyone knowing that you are the killer. You eliminate them easily one by one when they split, searching for the gold, but it's way cooler to take time to talk to everyone, win their trust, and confront them with each another. If you're good, you might even talk them into killing each other! It's not a difficult mission, but a very pleasant one indeed.
When all of the guests lie dead, go back to Ocheeva to get another reward. This time it's a blessing that constantly increases some of your stats.