In Elden Ring: Nightreign to search for other mud keys a bit compared to the original Elden. They are still needed to open prisons, but you can only find them at certain concerns. PC Gamer portal speakWhere to find mud key and why they need it.

The best way to extract the mucus keys in Nightreign is to search for special chests that look a bit more luxurious than conventional wood. They can be found behind the altar in most churches and on the top of the fortress, up the stairs becomes the place where the Boss knights often wait. The chance of reducing a key is not 100%, but it happens more often.
After the victory over the last boss Nightreign, the game will allow you to buy a unique monument in Bazaar, ensuring the player a mucus key at the beginning of each explorer.
Maggical locks are necessary to open prisons – they are marked on the map by round symbols. And the prison is a great way to form small chambers, because, depending on the selected complexity, winning they bring from 10 to 20,000 runes. If you suddenly find a prison in an ancient dragon or a death bird hiding, you can earn up to 30,000 runes – but victory over these monsters will be difficult.