10 Star Trek Characters You Didn't Realise Died Horrible Deaths

After the crew of the USS Discovery got transported into the mirror universe at the end of the episode Into The Forest I Go, one of the biggest mysteries was what happened to their mirror universe counterparts. Did the mirror Discovery and its crew switch places with the prime Discovery and end up in their universe like what happened with the mirror universe counterparts of Kirk and the others when they crossed dimensions in Mirror Mirror?

This was finally confirmed when they returned to the prime universe and made contact with Starfleet in the episode The War Without, The War Within. After forcibly boarding Discovery and getting Sarek to mind meld with Saru to confirm the crew's identities, Admiral Cornwell explained that they had thought Discovery was destroyed by Klingons nine months prior (or shortly after Discovery crossed universes). Saru explained to her that it was actually the ISS Discovery that they had seen destroyed.

What this means is that the mirror universe Discovery (under the command of the infamous Captain Killy) most likely got ambushed by Klingons shortly after switching universes and were overwhelmed by their power, probably because the Klingons in the mirror universe were so weakened by the Terran Empire that they posed no serious threat. The ISS Discovery likely thought they would be easy targets, a miscalculation which led to the deaths of everyone onboard (though unfortunately we never got to see this onscreen).