While the Elements of Harmony gave peace and harmony to Equestria's ponies, the Dark element only corrupted the ponies and scattered darkness to Equestria's people. Due to its immense Dark powers the ancient ponies used the power of the Elements of Harmony to seal the Dark element away inside of a temple, in order to make sure that it would never harm anypony again.
At some point in time, the Dark element was eventually discovered by a Mysterious pony.
The Dark element supercharges any creatures who absorbs its dark energy, making them stronger, faster and more powerful, but also corrupts them. It turns pain into pleasure, rendering any blows inflicted on them futile and heals their wounds as well. The dark element turns those who uses its dark energy into a creature desperate to do anything for more power and eventually, the user evolves into a beast-like state.
It is said that if a creature with an evil heart were to awaken and take it's dark power, that entity would gain enormous abilities. It can also give immortality.
When its Dark powers was absorbed by the Mysterious pony, the Dark Element disintegrated.