Reunions

Reunions is a main story quest and achievement in Fallout 4.

Detailed walkthrough
Dogmeat finds the scent of Kellogg, and leads the party out of Diamond City, stopping at a small pond to the south. There is another San Francisco Sunlights cigar which needs to be picked up and shown to Dogmeat to continue the search. He follows the trail along the railroad tracks, where mole rats and other creatures may be encountered. At the underpass, Dogmeat will stop in a doorway. In the nearby area there are signs of Kellogg's blood, and showing the bloody rag to Dogmeat will continue the search.

The trail continues across a bridge to the remains of an assaultron that can be spoken to for more clues. Depending on which way Dogmeat leads the party through the forest, there is a possibility they will run into a deathclaw sleeping in a small cave. On the outskirts of Fort Hagen, there are bloodied bandages on a fence that are the next clue for Dogmeat to follow. Eventually the trail leads to the entrance of the fort where the doorway has been blocked.

Enter Fort Hagen, either by a roof hatch, or through an exit door in the underground parking lot. Inside the facility are numerous synths and automated turrets guarding the building. The energy weapons bobblehead can be found in the kitchens of the command center. Deep in the facility, Kellogg begins talking to the Sole Survivor via the building's intercom, advising them to turn back. Eventually the Sole Survivor comes to an office with a sealed door where Kellogg concedes that the Sole Survivor won't leave and opens the door so they can talk. During the conversation, Kellogg will reveal that Shaun is being held by The Institute, and thus out of reach.

The conversation will end with both parties stating that the other must die and a fight breaks out. After Kellogg and his bodyguards have been killed a nearby terminal will reveal clues on where to continue the search for Shaun. The password for the terminal can be found on Kellogg's body, along with his cybernetic brain augmenter which is needed for the next quest Dangerous Minds, and his unique gun, Kellogg's pistol. The terminal will unlock the security doors which leads to the exit. Note that a doorway which led into this final area will be closed and remain locked once the battle with Kellogg is started, preventing backtracking into the fort; the exit elevator becomes available at the opposite end of the same hall.

Upon exiting the fort, a short scene plays where the Sole Survivor witnesses the Brotherhood of Steel arrive in the Commonwealth in full force, having traveled from the Capital Wasteland with the Prydwen acting as their mobile base. Several vertibirds will escort the Prydwen as it travels across the Commonwealth. The Brotherhood will announce their arrival via a loudspeaker and state that they are coming with peaceful intentions. This starts the quest Reveille if you've not dealt with the Brotherhood before, or Shadow of Steel if you have completed the Act 1 quest arc for the Brotherhood.

Back at the Valentine Detective Agency, a conversation with Nick Valentine and Piper Wright will complete the quest. If Nick was your partner against Kellogg, after speaking with you, he will direct you to talk to Piper at Publick Occurrences, where the next major part of the main storyline will begin.

Bugs
After using Kelloggs terminal a bug happens where any robot companion you have join you has their name reverted to Automatron. And cannot be changed. No known fixes other than reverting to a previous save.

After leaving Kellogg's house and you show him the cigar he will not move so you can follow him. The quest marker will remain above him but he will not go find Kellogg.

Reuniones Воссоединение (Fallout 4)