Project Horizons was designed by Goldenblood and the O.I.A during the war. The project was linked with EC-1101 in that it would automatically launch if it did not make contact with EC-1101 within a certain period of time.
Over the course of the story Blackjack continued to hear the name Project Horizons but could never find any information about it. Unlike the other wartime projects there was neither anypony alive to tell her of the project, nor any records to be found during her journey.
If any records of Horizons did exist, they may have been located within the O.I.A hub located within the core of Hoofington. Blackjack was never given the chance to thoroughly explore the building as she came under attack by Steel Rain and his soldiers.
It was only after meeting Cognitum in Chapter 65 that Blackjack learned the nature of Project Horizons; it is a powerful weapon which, when launched, would send a rocket, loaded with thousands of pounds of Moonstone, from its base on the moon and send it crashing down into the core of Hoofington and the starmetal chamber of the Tokomare. The moonstone would react violently with the starmetal that lined every building within the core and contained within the Tokomare itself, creating a powerful explosion of soul-energy, radiation and ennervation which, Cognitum stated, wouldn't quite destroy the planet, but would certainly be more than enough to destroy all life upon it.
Because Goldenblood was very good at political-spin and keeping secrets his exact reasons for creating Project Horizons are currently unclear. Those few who knew of the project believed it was to be a powerful super weapon to be used -either against Equestira or its enemy nations- should Princess Luna be dethroned and somepony (or zebra) else came to power. The main threat -supposedly- was the Ministry Mares who were in perfect positions to perform a coup and take the rule of Equestria away from Luna.
The project was one of two -The other being the Gardens of Equestria- that Luna had no knowledge of. She tried, and failed, to gain information on the project after Goldenblood's arrest for treason.
Others believed it was to be a new super-weapon which would end the war by making the Zebra nation unwilling to attack Equestria for fear of the weapons use. Blackjack believed this theory to be incorrect as the sheer destructive power of the project meant that even its owners would be afraid to use it.
The project had its roots in Project Starfall when Trottenheimer and Goldenblood noted the powerful reaction which occurred when even the smallest amounts of Star Metal and Moonstone came into contact with each other.
With his position as head of O.I.A Goldenblood had access to all the resources of the various ministries for his secret projects. For Project horizons he mainly utilized the resources of the Ministry of Wartime Technology and it's subsidiary companies.
From Robronco he took Mr. Horse's Mechasprites. Believed to be a failed prototype that was placed into storage pending further research and development Horse never discovered it was Goldenblood who sabotaged his demonstration of the machines, which would have otherwise worked perfectly.
From the defunct Equestrian Space Program he took the rocket technology. Using these rockets, Goldenblood surreptitiously sent the Mechasprites to the moon.
Upon the moon the Mechasprites fulfilled their programming perfectly and constructed a base from which the moonstone-filled missile would be controlled and launched. The Mechasprites cannibalised the rockets which sent them to the moon to build the missile and the controlling computers which would wait for contact from EC-1101, or to launch the missile after an absence of unknown time. Moonstone -of course- was naturally abundant on the moon.
*The moonstone which would be sent to earth was code-named Tom.