Outer Heaven is both the concept of Big Boss's vision (which is based on his interpretation of his former mentor The Boss's will which said soldiers shouldn't be treated like tools) and a private military company named after the idea. Outer Heaven is the main antagonist of Metal Gear and is presided by it's forger, Big Boss.
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By the time of 1995, Big Boss, who was also the leader of FOXHOUND, had established Outer Heaven as a nation in a large fortress stationed in South Africa. Said fortress was designed and built by the architect Kyle Schneider who later formed a revolution against the nation when his wife and child died. Big Boss also had his nation build the TX-55 Metal Gear to act as their ultimate weapon.
When the West noticed the tyrannical nation, they sent the rookie FOXHOUND operative Solid Snake to inspect it and rescue another FOXHOUND agent Gray Fox who was kidnapped by Outer Heaven and made a prisoner of war. Big Boss sent Solid Snake on said mission in hopes that he would fail, as he was only a rookie and he had an army of mercenaries sent to assassinate him. However, Big Boss was incorrect as Snake killed all of the mercenaries, saved Gray Fox, and destroyed Metal Gear. When the fortress was set to self destruct, Big Boss encountered Snake in person and battled him, only to be defeated by the rookie who escaped the facility just before detonation.
However, Big Boss somehow survived the explosion and went on to rival Solid Snake again in 1999 with his newly formed military known as Zanzibar Land.