Les Enfants Terribles is a secret government project that took place in Metal Gear, arguably the most important event. Crudely put, it is to clone Big Boss, the perfect soldier, for later use.
In 1971, Zero initiated this project to clone Big Boss. Big Boss did not approve of this, regardless, the project went on. Many characters were involved in this project including, but not limited to: Para-Medic, Revolver Ocelot, Kaz, and Eva.
This project produced 3 successful clones: Solid, Liquid, and Solidus Snake. Solid and Liquid were produced inside Eva. They were designed to be inperfect clones, so neither of them are identical to Big Boss. Unlike the two, Solidus Snake is an exact replica of the Big Boss. But soon after Solidus, the Patriots deemed the project as failures, and they were right, since all 3 clones turned out to go against the Patriots.
This program is on of the reasons why Big Boss left the Patriots and ultimately ended the series. I hope you liked it,