The Devil's Due

The Devil's Due is a side quest in Fallout 4.

Detailed walkthrough
A guard in Diamond City will say that something weird is going on at the Museum of Witchcraft, and the quest 'Investigate the Museum of Witchcraft' appears as a miscellaneous quest. The map marker leads to the body of Private Hart. However, it's not necessary to pick up this quest to do the rest of this questline.

On the exterior west side of the Museum of Witchcraft is a body, on which is Private Hart's holotape. Upon listening to the holotape it will be clear that Hart and her crew were running from something and took cover in the museum. Enter the museum via the cellar door a few feet away from the body. While in the museum there is growling and body parts will drop from holes in the ceiling periodically as one makes their way through. Head up to the first floor.

In the room behind the closed door there is a wandering savage deathclaw. Kill or sneak past the deathclaw and look around the floor in the rear bathroom to find Sergeant Lee's holotape (required to advance the quest) by his corpse. Around the body are some cracked deathclaw eggs and the pristine deathclaw egg required to advance the quest.

One then has the option to continue the delivery of the egg to Wellingham outside the Colonial Taphouse in Diamond City, where they will be awarded 200 caps (plus an additional 100 with every successful Persuade) and, after an additional dialogue choice, they will receive the Wellingham's recipe note which teaches the recipe Deathclaw Wellingham. Although not noted at the time, it also adds an additional recipe for tasty deathclaw omelette.

Alternatively, one can return the egg to the nest west of Parsons Creamery where a friendly deathclaw will appear and one can find a deathclaw gauntlet next to the nest. Finally, it is possible to take the gauntlet without returning the egg, allowing one to sell the egg to Wellingham normally. The friendly deathclaw will turn hostile if one tries to leave the area without returning the egg. One can still fast travel while the deathclaw is still friendly. Simply grab the gauntlet and fast travel to Diamond City without leaving the immediate area which would make the deathclaw hostile and prevent fast traveling.

Behind the scenes
After the original questline for Salem was cut, Liam Collins, one of the newest quest designers on the development team at the time, was given control of the Salem quest. He was determined to keep something at the Museum of Witchcraft before he and other designers came up with the concept of a Deathclaw being trapped in a building.

Bugs

 * The friendly deathclaw protecting the nest will be hostile if the Sole Survivor is wearing a power armor helmet with the targeting HUD installed. Putting the helmet on after completing the quest by returning the egg will immediately turn the deathclaw hostile.
 * Berry Mentats will also trigger a hostile status.
 * If one delivers the egg for the reduced reward having stolen it back from the nest a pristine deathclaw egg may remain in the player's inventory.
 * Occasionally, the quest will get stuck on "Listen to Sergeant Lee's holotape" (or Private Hart's) and will not progress even after listening to the holotape. This may occur if one enters the museum before they listen to the tape.
 * When the egg is sold to Wellingham, Private Hart and Sergeant Lee holotapes can become stuck in the inventory (try transferring tapes to Dogmeat, then putting it back into the workshop). The console command  also gets rid of the tape.