The Mirror Universe Trilogy was a miniseries of three books written by William Shatner with Judith and Garfield Reeves-Stevens, including Spectre, Dark Victory, and Preserver. The stories involved a crossing over into the Mirror Universe by protagonist James T. Kirk, with his main antagonist being the deposed Emperor Tiberius, his mirror counterpart.
Trilogy featured mirror universe versions of many TNG, DS9 and VOY characters, following the story of the DS9 mirror episodes featuring the Klingon-Cardassian Alliance. The characterizations here are somewhat different from those characters' iterations in other mirror universe storylines not written by Shatner, such as TNG novel: Dark Mirror, the Dark Passions miniseries and the current Mirror Universe anthologies and comics.
The Trilogy was not released under the banner of a separate miniseries, but the three books were given a name by the ST reference: Voyages of Imagination. This story should not be confused with The Mirror Universe Saga, an eight-part graphic novel set after Star Trek III: The Search for Spock.