The Story Behind the Crest of Darkness
The power "Darkness" appeared about the same time as the Light and the Rulers appeared. Some powers were forbidden to be researched by the Rulers. But there were people hated to be limited, so they kept on researching them. Since they had to do the research in the dark, they named the power "Darkness".
One key point is that, Darkness does NOT equal Evil, but its research is not forbidden for no reason. These powers tend to be powerful and irreversible (like nuclear bombs for now). And since it's so powerful, it becomes the first thing the Evil wants to get, and most of the time, they did. As time went by, people started to equalize these two.
In contrast to other power, everyone can active the Darkness power, but not everyone is able to handle it. (For other powers, only those who can handle the power can active it.) And if you let someone who can't handle the power use it, it spells disaster. The researcher sealed part of the power into a device, using it to test if someone is able to handle the power or not.
Only those who can handle the power the most comfortable can become the researcher of next generation. The device is the Crest of Darkness. But nowadays, nothing other than this crest is left.
As you can see, this crest is not originated from the Digimon World. There are numerous theories on how it got there. The most reliable theory is that about 20 years ago, when the Internet, as well as the Digimon World was in its starting stage, and was in construction. The miners are scanning the ground for mines, and when they scanned through the crest while it's underground, it released some of its energy. And the energy went through the scanner, and went into the Digimon World.
The builder of the world was scanning through tons of data to find suitable material to build the Digimon World. And when this flow of energy comes through to him, he found it good. And he used the flow of energy to replicate a Crest of Darkness for Digimon World.
A point is that the real Crest of Darkness is yet to be found. Although the replica only consists of a tiny little part of the original power, it is still considered to be very powerful, for people nowadays tend to have no power of the kind at all.
Digimon Evolved by this crest: Devimon -> Myotismon