Majora's Mask
"The troubles caused by Majora's Mask were so great... the ancient ones, fearing such catastrophe, sealed the mask in shadow forever, preventing its misuse."
A fierce and evil mask from an ancient tribe. The Skull Kid stole it from the Happy Mask Salesman, and it's evil overcame him.
Ancient Times
"But now, that tribe from the legend has vanished, so no one really knows the true nature of the mask's power...
...But I feel it."
After Link gets the Ocarina of Time back, he learns the Song of Healing from the Happy Mask Salesman. After Link does this, the salesman will tell Link the story of Majora's Mask. He tells of an ancient tribe (the tribes name is unknown, though it may be related to Odolwa) that created the mask. It was created for evil and "hexing rituals". But it got out of hand, and the mask caused many problems. The ancient ones decided to seal away the mask forever. But of course, there is no such thing as forever.