This is an extremely short piece, more brooding than action. In fact, Kull never draws his sword at all. Only Brule engages in any slaughter.
Howard chose to open this tale with a quote from Poe, and it's quite appropriate. Kull is burned out when the story opens, in what Howard describes as "the time of great weariness", and what would be called today a midlife crisis. (I'm looking forward to my midlife crisis and getting a Harley and a hottie, but I will probably ease into it slowly with the one that requires the least maintenance. That would be the Harley.) Instead of grabbing a wench and a fast horse and hitting the road, Kull merely broods about the meaninglessness of life and how nothing satisfies him now. While Howard wasn't fond of religion and the church, I have to wonder if he had been reading the book of Ecclesiastes when he wrote this. Howard describes Kull's daily routine as "an endless, meaningless panorama", much like the author of Ecclesiastes describes life as "vanity of vanities, all is vanity."
When no one is around, a servant girl suggests to Kull that he visit the wizard Tuzun Thune. Whereas Conan would have probably ravished the girl, Kull merely follows her suggestion. Tuzun Thune tells Kull to gaze into his mirrors and become wise.
The first mirror shows the past, and the savage battle for survival against flying dragons and other beasts. It's a world of endless struggle, with Death the only certainty. The second mirror shows the future, in which Atlantis and Lemuria are beneath the waves, and only their mountaintops remain, islands in the vastness of the ocean. Valusia and the other Seven Kingdoms are gone and forgotten, all their splendor and treasures dust. Tuzun Thune says this is the way of the world, one tribe supplanting the previous. It's all very depressing.
Then Tuzun Thune has Kull look in a third mirror. Kull sees only his reflection and wonders who the man is who gaze matches his own. He once knew him Kull begins to wonder who is the man and who is the reflection.
Kull visits Tuzun Thune every day, staring at his reflection in a mirror. He becomes more and more fascinated by the world in the mirror and wants to know what he would find if he passed through to the other side. He is in the process of doing so when when Brule shatters the mirror. Kull comes to his senses to find the lifeless body of Tuzun Thune on the floor before him, Tuzun Thune's blood dripping from Brule's sword. Brule informs Kull that he is the victim of a plot by one of the other nobles, a plot only discovered that very day. The servant girl who told Kull to visit Tuzun Thune is in on it. She's on the floor covering in fear for her life while this exchange between Kull and Brule is taking place. Amazingly Kull says she was merely a pawn and lets her go unpunished. After the girl tells Kull about Tuzun Thune, Howard describes her this way: " the smile of her scarlet mouth was cunning behind Kull's back, and the gleam of her narrow eyes was crafty." That doesn't sound like someone who was a pawn to me. We know she and Tuzun Thune were both members of the Elder Race, who once ruled Valusia. Conan would never have dismissed her this way, although he probably would have let her live.
In a letter to Tevis Clyde Smith sometime in February 1929, Howard lists all his sales to date. He records that he got $20 for "The Mirrors of Tuzun Thune" and that it was "more of the Shadow Kingdom, occult and mystical, vague and badly written; this is the deepest story I ever tried to write and I got out of my depth." [Collected Letters Vol. 1, REH Foundation Press, 2007, p. 311]
This is a deep story, but I don't agree with Howard's assessment. It's not badly written at all. Some of the paragraphs are quite powerful in their descriptions and mood. Howard was in his early twenties at this time. It's wouldn't be unusual to feel that sense of weariness he describes. Here's a young man who is trapped in a small town where no one understands him. He had to wonder at times if his desire to write was worth it. I spent part of my adolescence in a small town about fifty miles from Cross Plains, and I can tell you that what Howard describes is a very real sensation. Anyone who doesn't conform to the lowest common denominator expectations of society in those towns will sooner or later experience the fatigue (the weariness) that comes from trying to be your own person when all you meet is opposition and exclusion. Instead of being out of his depth, Howard seems to me to have poured out his feelings and his experience in this story.
I think he nails it perfectly, and that's why for all its brevity, this is a major story in Howard's oeuvre.