Reunions

Reunions is a quest and achievement in Fallout 4.

Quick walkthrough
Search for Kellogg with Dogmeat's help. Once found in Fort Hagen, confront and kill him. Search for more information and discuss this with Nick Valentine and Piper.

Detailed walkthrough
Dogmeat has the scent of Kellogg. Follow him out of the Fens, exit Diamond City, and head to the small pond to the south. Once picking up the cigar next to where Dogmeat stops, he follows the trail along the railroad tracks. Search the freeway underpass for signs of Kellogg's blood. Prepare for more four-legged fiends; the Sole Survivor encounter wild mongrels farther along the railroad tracks.

The trail continues across a bridge. Fend off a shambling horde of feral ghouls. Locate the remains of an assaultron and speak to it. Depending on which way Dogmeat leads throught the forest there is a possibility to run into a deathclaw sleeping in a small cave. The Sole Survivor heads up into the woods to the perimeter of Fort Hagen, where more mutated wildlife roam. Search for bloodied bandages. Then run to the exterior of Fort Hagen, where the trail goes cold at the blocked entrance. Be wary of the machine guns turrets on the roof.

Enter Fort Hagen either by a roof hatch or by an underground parking lot exit door. Once inside, prepare to face numerous synths, humanoid machines that use laser weaponry. Also beware of laser turrets; check nearby terminals to shut them down or use the turret override holotape if having it. Be sure to grab the Energy weapons bobblehead in the kitchens of the command center. Use the elevator to descend to the basement level. Expect fierce resistance from the synths. Access the command center and search for a wood-paneled office. It's here that Kellogg lets the player character in to face him.

Deal with Kellogg and his synth bodyguards, and don’t leave the area without Kellogg's terminal password or his cybernetic brain augmenter. Exit via a nearby elevator after accessing Kellogg’s terminal and unlocking the security door. One can also grab Kellogg's pistol here as well. Note that a doorway which led into this final area will be closed and remain locked once the battle with Kellogg is started, preventing from backtracking into the fort; the exit elevator becomes available at the opposite end of the same hall.

Upon exiting the fort, a small scene will play where the Brotherhood of Steel will arrive into the Commonwealth in full force, with vertibirds flying around the Prydwen with a voice saying that they were coming in with peaceful intentions.

Back at the Valentine Detective Agency, speak to Nick and Piper to complete the quest.

Bugs

 * After completing the quest, the Shadow of Steel may not trigger.
 * After killing Kellogg, the Prydwen may not be seen in the sky and Military Frequency AF95 may not play. This can be fixed using the console command
 * Once Dogmeat has found Fort Hagen, he may become stuck at the front entrance until entering Fort Hagen.
 * A game breaking bug maybe encountered while following Dogmeat, causing the game to crash to desktop after finding the bloody rags.
 * If attacked while talking to the Assaultron, the quest may bug and never progress past this objective.
 * Upon entering the command center, Kellogg's synths may attack and prevent completion of "Confront Kellogg". This seems to be tied to wearing power armor when nearing the confrontation with Kellogg, but triggers earlier in the general area so that reloading an older save and exiting the power armor early in the basement level may be necessary to work around it.
 * During the discussion in Nick Valentine's office, the conversation may not finish and one will be unable to complete the quest. Loading from a previous save or restarting the game should fix it.
 * During the end discussion between Nick, Piper and the Sole Survivor, the conversation may repeat after deciding who to travel to the Memory Den with. Going through it again, or leaving the building, will not have any negative effects.
 * There is a bug where if you attempt to talk to Kellogg while wearing power armor he will become instantly hostile, a work around is to reload a previous save near the command center entrance and do not wear the armor while exploring. Taking off the armor near the encounter will not work.