New Eden is Joseph Seed's community from Far Cry New Dawn, and it returns in the Joseph: Collapse DLC for Far Cry 6. Joseph built it after the nuclear Collapse at the edge of a Bliss zone.
In New Dawn, New Eden is a village with a church, a waterwheel and a shrine to Joseph. The New Edeners keep tamed cougars and still expect Joseph to return. The community starts out wary of outsiders.
The Far Cry 6 DLC revisits Joseph's past through his own mind. You play as Joseph and collect the three pieces of his broken cross. New Eden is one of the places from his life that you see again.
You do not need to have played New Dawn to follow Joseph: Collapse. Knowing New Eden does add weight to what Joseph sees in the DLC.