The Slavery Debate is an American event, and it is one of the most important events in the game.
It starts the debate, that will eventually lead to John Brown's Last Raid and Dred Scott v. Sandford, and will ultimately result in the American Civil War. Once this event has happened, it is no longer possible to stop the American Civil War.
It can happen in USA if slavery is allowed.
It has a mean time to happen of 20 months before 1845, of 10 months before 1855, 5 months before 1860 and a mere 1 month after 1860.
It adds the modifier slavery_debate, which enables all of the decisions and events related to the civil war. It also makes it impossible to change the slavery reform once the event is fired, so it is impossible to prevent the civil war, once it is fired.
On the other hand, if one manages to abolish slavery before the event, the Civil war will certainly not happen.