The speedrunner Christian Licht, better known as "Distortion2", solved the riddle of Bluepoint Games' new Demon's Souls remake together with some experts from the Soulsborne community. He was the first player who entered a locked area, which was added to the game by the developers, and his reward was a new boss armour set.
Distortion2 had to collect a total of 26 Porcelain Coins, which players discovered in the remake's newly added "Fractured World" mode previously. To find these rare items, you have to search the five game worlds meticulously, since they only appear in Pure Black or Pure White World Tendency (in mirrored mode that is).
To give you an idea: In Demon's Souls there is an underlying game system that lets the world react to the actions of the players. Depending on your current world state, you will face more deadly enemies or enjoy an overall easier playthrough (in her review, Silje explains the principle behind that procedure in a little more detail).
Back to the door mystery: You can find Porcelain Coins by ransacking specific, destructible objects in Pure White or Pure Black World Tendency. However, you cannot find the required amount of coins in just one playthrough, so you have to complete the game several times with the same character. When you done all of that, you need to trade at least 26 Porcelain Coins at once with the NPC Sparkly the Crow in the fourth world, the Shrine of Storms. Doing so will get you a Rusted Key which opens the door, that is located behind a new illusionary wall deep within Boletaria (1-3 Tower Knight Archstone).
This is how you get the armour of the "Penetrator" boss (whom you can see on the screenshots below). It is an unbelievable effort, but a nice action for the Soulsborne community, don't you think?