The Temple of the Union is a small compound of escaped slaves, who wish to create a haven to protect and inspire all runaway slaves.
The temple's leader, Hannibal Hamlin, dreams of a safe haven for would-be slaves, away from the slavers. Their respect of Abraham Lincoln is almost reverent, and they wish to set up their temple in the Lincoln Memorial - not only because it would be easier to defend, but because he is known to the slaves as the man who freed all from slavery in America. Because of this, Hannibal Hamlin will offer to buy any of Lincoln's artifacts.
There are several ways to get its marker on your map. You will be given the marker if you help out Mei Wong in Rivet City, or by successfully removing a collar from a slave in a random encounter. You can also find a map on the several corpses of dead slaves, presumably failed runaways.
It is a ruined two-story building that houses a group of escaped slaves. Initially, Abraham Lincoln's head is here, although it is unclear how it got to this location. It is possible that it was being repaired or was stolen. Completing the quest Head of State will result in the head being moved to the Lincoln Memorial.
A 'Type A' random encounter spawns in the interior of the Temple on the second floor, but only after the slaves have moved to The Lincoln Memorial or the player helped the slavers kill them.
There are a few skill books in the shacks on the elevated highway to the west.
There is a Vertibird landing zone on an elevated highway near the Temple. It occasionally drops off a number of Enclave soldiers, and battles often break out between them and the raiders who occupy the open shack on the highway.
There is a chance that you can encounter Talon Company mercs outside after entering a while earlier, prompting a firefight between Talon Company and the Slaves.
After talking to Hannibal, and not swearing your loyalty to the Temple, you are "locked" inside until you do. Hannibal doesn't seem to realize that the player can jump out of one of the many windows.
Dogmeat is able to open the door shortly after the player is locked inside the Temple.
Ready your weapon if you want to use fast travel to get to the Temple after the slaves have left or been taken care of; a mutated animal or a hostile robot will always spawn in front of you or near you. This may kill the caravan merchants as well.
If you are seen picking the average lock to the Storeroom, everyone inside the Temple will become hostile.
If you walk back outside after talking to Simone, before she shuts the gate, you will be locked out; Simone only replies that Hannibal is waiting and will not open the gate again. You can, however, ring the doorbell located under the "Ring Me" sign, or pick the gate (very hard lock).
Many of the items inside the temple are considered owned, even if the slaves move or are killed.
Hannibal will say that you cannot leave, however, nothing will happen if you leave anyway.
Ringing the bell after you have gained possession of a key will result in Simone telling you to open the gate yourself.
There is no noise when you ring the bell, pressing it once will be sufficient.
The inhabitants are named after members of Abraham Lincoln's cabinet. Hannibal Hamlin was Vice President, Caleb Smith was Secretary of the Interior, William "Bill" Seward was Secretary of State, and Simon(e) Cameron was Secretary of War. The dog, Four Score, is named for the opening words from Lincoln's Gettysburg Address (Four score and seven years ago, our fathers brought forth on this continent...).
The name "Temple of the Union" comes from the words above Lincoln's statue at the Lincoln memorial. The passage reads, "In this temple, as in the hearts of the people for whom he saved the union, the memory of Abraham Lincoln is enshrined forever.” On the block Caleb is working on, the words "Temple of the Union" are visible, with the others faded out. Presumably, the block arrived at the building at or around the same time as the statue's head.
The name of the Temple is based on the federal government of the United States during the American Civil War. (See: The Union)