The Renegade Shadowscale
This is an optional quest. It becomes available once you have completed the "Scheduled for Execution" contract and remains available until "The Purification" quest after you start working for Lucien Lachance.
When you speak to Teinaava, he will ask you to deal with a certain matter for him and it will have no effect upon your standing with the Dark Brotherhood one way or the other. If you accept, you will learn about a group of Argonian assassins, known as "Shadowscales". Teinaava, Ocheeva and another Argonian, named Scar-Tail, are such assassins. They were raised from birth to be Shadowscales. Scar-Tail has refused to perform his duties as a Shadowscale and must be eliminated. Teinaava and Ocheeva are not permitted to do this because they are constrained by rules very similar to the Five Tenets of the Dark Brotherhood. You, however, are not so constrained and can eliminate this renegade with impunity. To prove that the deed is done, you'll need to bring Scar-Tail's heart.
Scar-Tail is at a camp called "Bogwater Swamp", which is at the very southern tip of Cyrodiil, east of the Niben. It is not close to anywhere you would have gone for the Main Quest or Dark Brotherhood. However, you might have markers for Fort Blueblood (Mages Guild), Tidewater Cave (Nocturnal) or Darkfathom Cave ("Whom Gods Annoy"), which are relatively close. If you have been working on or completed the "Seeking Your Roots" quest, you might have stumbled across the camp since it's fairly close to the coastline.
When you get to Bogwater Swamp, you have a couple of options. Either kill Scar-Tail as planned or let him go and give Teinaava the heart of a different Argonian, which Scar-Tail can provide. Both ways will work and it's your choice on how to proceed. If you kill Scar-Tail, check the pile of rocks near the campfile to collect some gold. If you let him live, he'll tell you where the gold is and you're welcome to take it.
As long as you have an Argonian Heart to give Teinaava, he's completely satisfied and presents you with the "Boots of Bloody Bounding", which fortifies your Acrobatics and Blade skills.