I recently decided to go on a killing spree. A slow killing spree, but a killing spree nonetheless. Well my first kill in Markarth was Rhiada, Eltrys' wife. Seconds after killing her, a messenger arrives and says he has a letter for me. But he gives no description of the letter. He simply skips to "looks like that's it, got to go." No quest icon appears, but I look in my inventory and find a letter from Yngvar the Singer. So I hunt him down, and he makes no mention of it. Just keeps saying generic "what do you need" dialogue. I wait 12 hours and see if he reacts different at night ... nothing. Well, randomly, I kill Ogmund next, and on my quest screen it reads "report your success to Yngvar". So I go back to Yngvar ... and still, nothing. What's more amazing is that I have googled this like crazy, and looked it up on many Skyrim wiki sites, and have only found one mention of it in a random discussion in a similar fashion to this post.
I also got a courier arrive with a letter from Yngvar the Singer, and it was after I killed someone in Markarth. I went there to kill the chef after getting the identity of the gourmet for the dark brotherhood quest, and there were two other NPC's in the room with the chef.
My first attempt at killing the chef got me bounty, so I reloaded my save and tried again, this time I made sure the door was closed and I killed all 3 of them in sneak mode quickly so that they didn't make enough noise to alert the guards outside.
Suffice it to say, I managed to pull this off without getting any bounty on my head. However, shortly after leaving Markarth, I was in another area of the world (I can't remember exactly where now), and the courier showed up.
I haven't gone back to Yngvar yet, but I was planning on it. If I learn anything about it, I'll post here again. ;)
I dropped the letter and picked it up. A wierd message came up for me to visit Yngvar. He told me to assult ogmund, but not kill he. I got 50 gold when it was done. Odd little quest and screwed up language to it. Drop the letter and then read it and pick it up. @.@