Siege of the Citadel was a board game from Modiphius that took players into the world of Mutant Chronicles. As part of the Kickstarter campaign for the game, backers could get the Siege of the Citadel crossover book so they could use elements in their Mutant Chronicles RPG sessions. Now, that's available to the rest of us with the release of the digital version of the Siege of the Citadel Crossover Book.
From the website:
Previously only available to Siege of the Citadel Kickstarter backers, this digital release aims to bridge the divide between the Mutant Chronicles Roleplaying Game and the Siege of the Citadel board game.
The 83-page supplement presents rules and guidance that converts material from the board game for use in the RPG such as character sheets for 18 legendary characters including Lori Faust, Max Steiner, and Inquisitor Nikodemus!
Most of this PDF’s content can be used without any knowledge of, or components from, the Siege of the Citadel board game, but you will require access to the Mutant Chronicles Roleplaying Game core rulebook.