Innocence Lost

Walkthrough
When the Dragonborn first enters the city of Windhelm, a dark elf, Idesa Sadri may be speaking to Grimvar Cruel-Sea about a local boy, Aventus Aretino who has been dabbling in the dark arts. The elf warns the boy to stay away.

Talk to Aventus Aretino
The Dragonborn can then decide to seek out the boy and see what is going on.

Entering the Aretino Residence, the Black Sacrament being invoked by the boy can be overheard, standing in a small alcove next to a human skeleton surrounded by a human heart, an iron dagger, a copy of A Kiss, Sweet Mother and a nightshade flower.

The boy immediately assumes that his supplication has been heard by the Dark Brotherhood and that the Dragonborn is one of their agents, sent to help him. The boy then explains that the woman who runs the Honorhall Orphanage in Riften, Grelod the Kind, is a horrible old woman who must die.

Kill Grelod the Kind
The Dragonborn can then decide to help Aventus and travel to Riften and find Grelod at the orphanage and kill her. Stealth is a good option here, but there are no real consequences to killing the woman and the children at the orphanage even cheer. The only person horrified by the act is Constance Michel, though she only becomes frightened and cannot be talked to. She does not apparently report the crime.

Tell Aventus Aretino that Grelod is dead
Once the deed is done, the Dragonborn can return to Windhelm and advise Aventus. For this, the boy unhands his most valuable possession, a silver plate.

Verlorene Unschuld