The Thor is a new anti-air assault mech for the Terran race. Though it was historically constructed by SCVs on the field, it has been scaled down in size and is now made in the Factory.
The Thor has the longest ranged anti-air attack in the game. Though it was originally designed as an anti-ground weapon, its armament containing large particle cannons as well as huge Siege cannons, its role has been changed so that it did not overlap with the Siege Tank. It is now primarily an anti-air weapon, but also has a ground attack, similar to the Goliath of StarCraft: Brood War.
Despite its size, the Thor can be transported by a Medivac. It transforms into a smaller, cube shape in order to be picked up. The Thor takes up all 8 transport slots within the Medivac.
The Thor project has been shrouded in mystery since its very beginning. Agents of the Umojan Protectorate were the first to uncover a trail of clues that the Terran Dominion was intent on creating some kind of new terror weapon. The Umojans' curiosity was aroused when they heard rumors that the huge Simonson munitions facility on Korhal IV had been placed under a complete security lockdown by Dominion forces. Something big was in the offing, so big that the workers at the Simonson plant had been virtually placed under arrest to prevent any information from leaking out.
The crack team of spies swung into action to search for leads that would betray the work being undertaken. Hacked cargo manifests showed deliveries of battlecruiser-weight neosteel armor plating extending back over a year. Passive sensors that the spies planted around the facility picked up occasional industrial-strength electromagnetic discharges and regular seismic shocks. An increasing flow of high-ranking Dominion officers was observed coming and going, indicating that the project was close to completion. Yet beyond these tantalizing pieces of the puzzle, the Umojan team drew a complete blank.
The Umojans began to be frustrated by their lack of progress, and frustration can lead to recklessness. A young operative on the team, Ulli Trey, offered to take the risk and try to infiltrate the facility. The team leader was reluctant but out of other options, and so he approved the plan. Trey's approach was flawless: she liquidated the driver of the Dominion commander and took his place. As she had anticipated, the rigorous security checks needed to get through the outer cordon around the facility were focused on the high-ranking officer instead of his lowly driver.
Once inside the facility, Trey released a clutch of nanotech micro-spies to trail the commander as he went deeper into the facility. From this point Ulli Trey could only sit and wait, hoping that the stealth capabilities of her miniature spy drones would be good enough to prevent them--and her--from being caught.
Within a few hours Agent Trey was free and clear, carrying vital information about the Dominion's most significant military development since the Brood War: Project Thor. The micro-spy images showed a huge bipedal war machine armed with two giant particle beams and a dorsal artillery battery that wouldn't be out of place on a battleship. Most disturbing of all was the demonstration that this monster could be built from the ground up by ordinary construction vehicles in an astonishingly short time frame. The Umojan team reported back to Protectorate, thoroughly shaken by their findings and praying that some counter could be found for such a beast.
The Thor was originally so massive it had to be built by an SCV. It was available after a Munitions Depot was created. This was scrapped due to the fact that this method of construction did not limit production.
Additionally, the Thor originally was equipped with massive 280mm Siege Cannon. When used, it could deal enough damage to destroy a Planetary Fortress in a single volley (see Terran Gameplay Demo). This was later removed, due to overlapping roles with the Siege Tank, and was instead replaced with very effective, long ranged AoE anti-air missiles.