The Crystal Empire
The Crystal Empire was introduced in the Season 3 episode "The Crystal Empire: Part 1. It is described as being a place of powerful magic. It also serves as the home for a race of ponies known as Crystal Ponies. Crystal Ponies are similar to Earth Ponies, but have a crystally-coat and gradient hair. It is protected by the Crystal Heart, a powerful magical object which channels the love and happiness of its inhabitants.
1000 years prior to the show's setting, the empire was taken over and ruled by King Sombra, an evil unicorn who used dark magic to enslave the Crystal Ponies. To solidify his rule, he would hide the Crystal Heart deep within the palace and set up a number of traps to protect it, keeping the Crystal Ponies from defeating him. Sombra would eventually be overthrown by the combined efforts of Princess Celestia and Princess Luna. Sombra was turned into shadow and imprisoned within ice, but he was able to curse the empire, making it disappear for 1000 years.
When the empire suddenly reappears, Celestia sends Princess Cadance and Shining Armor to investigate. When Sombra also reappears, Celestia tasks Twilight Sparkle with finding a way to protect the empire from Sombra. When she arrives, Twilight finds that the Crystal Ponies are depressed and amnesiac, causing their coats to appear dull. After a series of adventures, Twilight finds the Crystal Heart. When she becomes victim of a trap set by Sombra, Spike and Princess Cadance are able to place the heart in its resting place. The Crystal Ponies are able to use their love and happiness to empower the heart, presumably killing Sombra. After this, Cadance is made Princess of the Crystal Empire, with Shining Armor helps her rule.
Later on, the Crystal Empire is in the running to be the location of the next Equestrian Games. Twilight, G4 Rainbow Dash, and the others are able to convince the selection committee to pick the empire as the next location; the Crystal Ponies' subsequent happiness sends another burst of energy from the Heart.
The empire appears briefly in the Equestria Girls movie. It is to be the site of a Princess Summit that Celestia, Luna, Cadance, and Twilight are attending, and is the home of a strange mirror that allows a pony to pass into another world.
The Crystal Empire serves as the name for a grouping of toys revolving around Season 3. Toys released under this name included updated versions of the Princesses and the Mane Six, as well as many other toys.