Midnight Delves GuideNulleaus Nemesis Boss Guide
The cap only applies to shards, and does not include any already restored keys.
Unlike The War Within, Coffer Keys no longer drop directly. Instead, players predominantly gain Coffer Key Shards as a currency, but they'll do so from a much wide range of activities, including quest rewards, caches, rare mobs, weekly activities, prey hunts, and more. There is a weekly cap though, as players can only gain 600 shards per week. So far, it appears that:
Certain quests, including favors during Saltheril's Soiree award 25-35 shards.
Rare mobs award 50 shards each.
Prey Hunts award 75 shards each.
Weekly activity rewards, such special assignment world quests and Apex Cache award 100 shards.
Coffer Key Shards are now a currency, awarded from a variety of activities you're probably doing anyway.
That said, players can still obtain at least a couple fully restored coffer keys from Naleidea Rivergleam in the Delve HQ. Two are contained in the Delver's Starter Kit, and two more can be purchased outright once players are sufficiently progressed along the Seasonal Delve Journey. These keys ignore the shard cap, so there's no fear of wasting them.
After completing the The Season Starts: Midnight introductory quest, the Delver's Starter Kit tooltip will state that it gives 200 coffer key shards, though this is not actually the case and likely just a holdover from the previous system. We can confirm it continues to give two full keys that do not count against the weekly cap. The bag is limited to one per warband though, so purchase wisely.
Fully restored coffer keys are now much more limited.
Converting the shards to keys is easy, simply walk into a delve and it will be done for you! While shards are capped each week, Keys are not, so converting them will help ensure you continue earning shards towards the cap each week.
Alternatively, you can ask Naleidea Rivergleam in the Delve HQ to combine them for you, or purchase Coffer Key Glue.
