Thresh was once a Happy Chain Warden Child. He had Just did his job, and nothing other. His best Friend was Sombra, who was also a Chain Warden. One day, the Changelings attacked the Gateway they were protecting. Thresh and Sombra had both given it their best. Sombra had been in love with the Arts of the Shadows, while Thresh had been Training in the Void. Queen Chrysalis went through the gateway, and seeked to Kill the Chain Princess and Absorb her Beauty. Thresh and Sombra had chased after Queen Chrysalis, although... they failed. The Chain Princess had been killed, but said to Thresh and Sombra after death in her soul spirit "I knew my Death would come... either way. She couldn't absorb my form and beauty, however. What you both must do... you must Kill Chrysalis... it is the only way to revive me." After hearing the News, Thresh had been Delighted. The Chain Princess had a hope.
Thresh and Sombra had embarked on their quest, to locate news of Changelings. Not only would they Protect the Princess, but also all. They had protected the Sand Princess from the Changelings. Sombra immediately asked The Sand Princess if she knew anything about Chrysalis. "The Crystal Empire. If you had gotten Control of the Crystal Empire in anyway, she would come." Sombra set off to gain control of the Crystal Empire, leaving Thresh behind. Years Later, Sombra had gotten in Control of the Crystal Empire. He had kept control. Years after, Sombra started to believe the Sand Princess was lying. But, he was wrong. Chrysalis turned her attention to Equestria. She has been hunting there for Love to Feed on Ever Since.
After the incedent inside Canterlot, Queen Chrysalis had been blasted back to the Sands of the World. Sombra had been turned to Shadow 998 Years ago. Thresh had taken up the job for himself, and finished the job. He protected the Sand Princess, and Killed Chrysalis. The Chain Princess has offered to Marry Thresh, though Thresh said she should marry someone else; He said he was not worth the while.
Meeting the Main Ponies
He had heard of the News of Sombra coming back, he had been delighted. He had teleported in the Crystal Empire, and tore the page of the Crystal Heart, preventing them from gaining it. He had also sapped all of Cadence's Energy, so she could not hold up the energy for long. He had created the Blizzard outside of the Empire so the Ponies could be slowed down and possibly killed during the Blizzard. After the world was saved, he told the ponies "You've no idea of my history... do you? Twilight, you could look in your books... That wouldn't work. Princess Celestia would want to cover up her guilt. Let me assure you... Sombra has not been trying... neither have I. P.S. Chrysalis is gone. I took care of her long ago."
Movie of The Shadow Armor
During the Movie, in the beginning, Twilight and Friends are all in the Library, reading Ghost Stories for fun. Then, outside, only Spike Notices the Odd Storm. He confirms Twilight of it, and she Calls it "Storm of the Shadow Armor". Twilight explains that during these storms, images of the Shadow Armor will appear in the Lightning. Also, it is the only time you can obtain the Shadow Armor and Possess it forever. After Twilight Finishes, Thresh appears in front of the Ponies and states "The Shadow Armor... You're a Fan of Storms, right? I AM! Once the Shadow Armor is in my possession, I will be Invincible, immune to Spells and all that! The Everfree Forest... Just the Place Where it is! Would you like to join me? Hmm? Give me the Armor? Well! GOOD RIDDANCE!" "And here's a Map. I love a good Competition.