Broken Vows
Oblivion: Dark Brotherhood Quest
You'll find this quest in the dead drop located in the courtyard well at Castle Skingrad. Your next target is J'ghasta, a Khajiit in Bruma.
Head to Bruma and follow the markers to J'ghasta's house. Inside you'll find a secret hatch under some cloth that leads to his training room. Inside, you'll find this boxer ready to fight.
If you're still fairly low in level, a magically enhanced item will whelp you take care of J'ghasta quickly.
Once J'ghasta is dead, head to the Old Bridge, located southwest of the Imperial City to get the next orders and your 500 gold reward. Your next quest is Final Justice.