For the first time in series history, Diablo 4: Vessel of Hatred introduced a dungeon that locked out the friendless among us and the expansion's director now says it's unlikely to let them in.
Diablo 4's first major expansion bought with it a dungeon called the Dark Citadel, which is more akin to a Destiny or MMO-style raid that has specific rules you'll need to learn - either through trial and error or a guide - to complete. You'll also need at least one friend to join in co-op to even get into the area in the first place or you can use the game's party finder feature to squad up with strangers.
