In the mirror universe, Slar was a Gorn who lived during the 22nd century. He was employed as a slave master by the Tholians. In the year 2155, he was in charge of the slaves who were examining and disassembling the Federation starship USS Defiant after the Defiant had crossed over from the primary universe.
After an attack by the NX-class ISS Enterprise on the space station asteroid where Defiant was docked, the ship was captured by Starfleet officers from the Terran Empire. When Enterprise was destroyed, the Terrans took Defiant and captured the slaves. After hiding in the bowels of the ship, Slar's escape was revealed after the torture and murder of some of the slaves in interrogation. Slar murdered several of the Starfleet boarding party before they were able to locate him. Slar had made contact with the Starfleet crew and requested an escape shuttle in exchange for his stores of vital components to reassemble the ship's systems, but Jonathan Archer refused the parlay and sent search teams after Slar. He killed and injured included Major Reed, Ensign Kelby, and Corporals Brown, Scott and McKenzie, culminating in cruel tricks like baiting crew with needed spare parts and then killing them, and a particularly devious explosive baited with a communicator. Eventually, Archer entered into personal combat with Slar, trapping the Gorn using artificial gravity intensification. Archer executed Slar with a plasma rifle. (ENT episode: "In a Mirror, Darkly")
In an alternate reality, Slar was a member of the Imperial Elite, the super-powered enforcers of the Imperial Planets Science Police. In this reality, Slar wielded the atomic axe, a special weapon that could cut through any material with its sub-molecular blade edge. When encountering Slar, Chameleon Boy recognized the axe—in his reality, it was the mantle of the villain known as the Persuader.
When the Legion of Super-Heroes and a landing party from the Federation starship USS Enterprise were trapped in the Imperial Planets' timeline, the Slar version of the Persuader and the Imperial Elite squared off against the time travelers and were defeated. Slar in particular was defeated when Chameleon Boy shapeshifted to wrap around Slar's body where the Gorn could not turn the axe on himself, and Doctor Leonard McCoy was able to administer a hypospray sedative. Afterwards, when Shadow Lass was impressed at Hikaru Sulu's martial arts technique, Chameleon Boy grumbled about having faced down the atomic axe. (TOS - Legion of Super-Heroes comic: "Issue 3")
- It is unknown whether or not Slar has a counterpart in the primary universe.