A soul contract is a contract that can only be signed when both parties have absolute trust in each other. It is also the highest contract in the legend of the magic world.
Under the intervention of Salazar Slytherin, Severus Snape and Voldemort signed an incomplete version of the soul contract and were tied to the same warship.
In exchange, Severus Snape came to Tom Riddle's childhood, the orphanage full of gray memories, and the story begins.