The easiest way to find a guard is to go to the nearest city. The Dragonborn then needs to locate a guard and report her death. Once Astrid is dead, grab the Blade of Woe and the Abandoned Shack Key from her body. Instead of doing this, the Dragonborn can choose to kill Astrid. She then explains that the Dragonborn must fulfil a contract for the Brotherhood as recompense and indicates three captives who are blindfolded and bound, telling the Dragonborn that they must decide which captive is destined to be assassinated and kill them, but she will not tell you which one.
She explains that Grelod was by rights a Dark Brotherhood contract and that the Dragonborn has infringed on their territory. After killing Grelod the Kind and deciding to have a good night's sleep at the Sleeping Giant Inn, the Dragonborn wakes up inside the Abandoned Shack, with Astrid, who turns out to be the leader of the Dark Brotherhood.
