Diablo 4's Vessel of Hatred expansion is a firm statement that Blizzard is and will continue to update the game for the next few years. The DLC addresses some pain points from D4s rocky history from launch to now, such as more build variety for all classes, with Diablo 4 classes get new active and passive skills to give players more options. This is a good direction for the game, and it's something that Blizzard will likely continue to do over time, most likely because every part of the ARPG will improve on each of its pillars. Most recently, Vessel of Hatred gave a new spin to Diablo 4s multiplayer aspect with the Dark Citadel raid.
The fact that Dark Citadel as an activity even shipped is a good indication that the MMO part of Diablo 4 may not go anywhere, for better or for worse. Action RPGs and looters with multiplayer elements are not uncommon, but having some gear locked behind these activities can be frowned upon by the community and fans of the genre. Destiny 2 is a good example of this, and Diablo 4 follows in its footsteps with Dark Citadel.

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Diablo 4: Vessel of Hatred's Dark Citadel Multiplayer Controversy Explained
A reason why Dark Citadel comes in Diablo 4 not getting an overly positive reception, as noted in the pre-launch reveal, is that communication and group play are both required, essentially removing the solo play aspect of the ARPG for this specific activity. In fact, Dark Citadel cannot physically be completed solo as at least two players are required to open doors and place puzzles between encounters. This is a typical aspect of raids in MMOs and games like World of Warcraft or Destiny 2 is no stranger to this.
New data shows that over 85% of Diablo 4 players are primarily or exclusively solo.
Recently, Destiny 2Exotic Class Items were criticized for being a big part of The Final Shape expansion, but they were made available exclusively via a two-person activity. Although other methods of obtaining the items were available, the first one had to be unlocked through this multiplayer activity which required heavy communication between players. As such, it does not come as a surprise that Diablo 4 faces similar criticism for an activity that cannot be completed alone, regardless of difficulty. However, Dark Citadel is well worth it for those willing to play it, even if it's not all roses.
The Dark Citadel raid also comes with typical MMORPG accessories, like weekly rewards for clearing the activity and exclusive loot, which can be quite controversial and has proven to be just as much with other games. Another potential problem is finding a group, which can be done quite easily thanks to Diablo 4 Season 6's new party finder, but it would also be beneficial to have people of the same level of power and build to not ruin the experience for others. On top of that, Dark Citadel runs may eventually reward the coveted and equally problematic Scroll of Restoration.
The Scroll of Restoration restores Tempering attempts on a certain item, which has long been requested by fans due to how frustrating Tempering can be to spec in Diablo 4.
Why Diablo 4's Dark Citadel is worth running
Despite these issues, the Dark Citadel raid is a pretty unique piece of endgame content in the Diablo 4and arguably one of the best parts of the Vessel of Hatred expansion. It's not something every player will run, much like only a small percentage of the player base completes Destiny 2 Raider, but it's good content that challenges players to think strategically, solve puzzles and master their builds before running. The fact that players can't really carry others through the activity can be off-putting, as it's necessary to know what to do and get involved in the mechanics, but Dark Citadel is quite rewarding even beyond the loot it offers.