When Severus is outed as a spy when trying to save the life of Longbottom's , he is tortured and hospitalized for two years. When Albus is faced with the possibility of having to consent to stop the spells that are keeping Severus alive, he founds a solution in an unknown enexperimented potion of his. The result is not what he wants at all

Severus has to live again, he will choose differently, this time anyway