A merchant named Arahn Kimoi was found murdered in his manor, upon getting the latest rumor, the player gets the "Arahn Kimoi" topic added, and they can unravel the mysteries of his untimely death. The suspects are: Medina Sirolus, who didn't get along with Arahn and was seen threatening to "ruin" him. Ramus, Arahn's former servant who was fired for reasons he'd rather not discuss, and Brescis Telettain, who had on numerous ocassions refused to sell his business to Arahn, and Arahn was supposedly attempting to put out of business (and replace with his own) in retaliation. The player can talk to all of these witnesses, as well as the Halgerns and Nerilius Ludos - the guard captain - to get clues for. This eventually leads to the player discovering that an outside force had assassinated Arahn, a force that had contacted the Dark Brotherhood to carry it out. The player goes to the location of the meeting place, where the Dark Brotherhood member were to have been waiting. The meeting place is a room in Gosha Inn (the other one, not the one that's up for rent - but the renting function isn't implemented at this time I don't think). You can ask Foroch to unlock the room for you, considering there are others who've done the same, he doesn't question it. Open the door and find the Dark Brotherhood member dead, likely assassinated. The quest ends there, with no way of knowing who was responsible for this assassination. Though if a player is a member of the Cyrodiilic Brotherhood, they can then follow this quest up with the Dark Brotherhood's own investigation, leading to a semi-high profile individual in Sutch who is affiliated with the Kali Mes. Talking to Thelius or Nerilius will give you your reward, depending on which you decide to go to, depends on your reward.
These are the following quest paths:
Path 1. Get the rumor about Arahn, then talk to Nerilius to start the quest. Investigate the suspects (Brescis, Medina, Ramus), to get the main gist, speaking to Brescis' apprentice - Ris gra-Kerrar, Thelius Halgern, and his wife Salonia, may also yield some valuable information. If you speak to Thelius, he'll ask that you bring the DB member to him, accept, and head to Gosha Inn. Find the murdered assassin, loot his corpse, bring the murder weapon back to Thelius for your reward.
Path 2. Get the rumor about Arahn, then talk to Nerilius to start the quest. Investigate the suspects (Brescis, Medina, Ramus), to get the main gist, speaking to Brescis' apprentice - Ris gra-Kerrar, Thelius Halgern, and his wife Salonia, may also yield some valuable information. If you speak to Thelius, he'll ask that you bring the DB member to him, decline or accept, and head to Gosha Inn. Find the murdered assassin, loot his corpse, bring the murder weapon back to Nerilius for your reward.
You shouldn't 'need' to speak to Thelius, or even so much as enter Halgern manor at all, to complete the quest. The DB member should be in the room (dead) once this quest is started, you will need to make the npc yourself. Arahn's corpse is already made, don't forget to attach a new script to it for this quest claim. A rumor going around afterwards about dark brotherhood assassins from Kvatch might be interesting in Brina Cross and/or Gosha Inn (or one rumor for each) after you finish the quest, but is strictly unnecessary. You will need to make the DB member corpse.
The latest rumor relating to the quest is already implemented. Please make sure to add a journal condition to it in order for it to stop appearing once the player finishes the quest (feel free to add an additional rumor if you want about it, but any additional latest rumor dialogue is entirely optional and should only be added if you feel that it significantly enhances the experience. Sometimes less is more).
"Did you hear? Arahn Kimoi was found dead in his manor last night! I here talk of assassination, but Lord Halgern and Nerilius Ludos are trying to keep a tight lid on the situation."