Also known as the Source, the Serum was created by the scientists of the South East Asian Republic as a response the American and European Super Soldier Serum projects. They succeeded beyond their wildest dreams.
The Serum is a bright green liquid that is taken by injection, which mutates those who take it into terrifyingly powerful superhumans, so different from baseline humanity that they consider themselves another species (although it is unclear whether this is true or merely rhetoric). Several thousand of them already exist in the S.E.A.R. and they have created their own realm and invited any who wish to join them to do so. So confident are they in their power that when Nick Fury sent Hawkeye to steal a sample, they actually gave it to him.