The mirror universe was introduced in the original series during an episode called ‘Mirror, Mirror’.
The mirror universe was shown several times in Star Trek: Deep Space Nine where the actions of the original series crew had left lasting effects on the universe.
Jonathan Frakes has just revealed a pretty exciting Star Trek: Discovery spoiler – though sadly it’s not a Riker cameo.
Frakes will be directing an episode of Star Trek: Discovery, which is probably how he came by the information.