Deep in the bowls of a Dwemer ruin, on my way to Blackreach. It was very dark. J'zargo and I had just finished a battle with a few Falmer Stalkers and everything was quiet again.
We crept forward and I noticed some stairs to my left. So, I headed there, thinking J'zargo is right behind me.
As I rummaged through the things on a desk, in a small and very dark alcove, all hell broke loose. We had been assaulted by a Falmer Shadowmaster and a couple of Nightstalkers. My heart beat so fast and I was swinging my blades for dear life.
After everything quiet down again, I whispered J'zargo's name, but there was no reply. I thought I may have lost him earlier and he would show up eventually.
So, I made my way into Blackreach alone and rested a few hours in the first dwemer building you come to. No J'zargo. So, I took the elevator up to Skyrim, went home, got my Husband Vorstag and we retraced my steps.
And there we found him... lying in the dark... dead.
This was my scariest and my saddest moment in the game.

(I had to use the console to rez J'zargo lol)