Assassin's Creed Unity can be downloaded for free from the Ubisoft website. The French company has decided to make available to everyone the chapter of the saga set in Paris, this to remember Notre-Dame before the regrettable fire.
According to what is described on the site, Ubisoft's will is to remind everyone of the beauty and majesty of the Parisian cathedral, especially to people who unfortunately can no longer see the cathedral in all its real splendor.
Assassin's Creed Unity for PC is downloadable free of charge here. Ubisoft stressed the desire to help in the reconstruction of Notre-Dame, if you want to take part in the reconstruction you can make a donation to Ubisoft.