What controversial topic did the Scott v Sanford case deal with?

Slavery.

It was meant to be a simple verdict about whether a slave, Dred Scott, who had been taken on to free soil and then back into slave country, could sue for his freedom.

The controversial part was the reason given for refusing Scott's request. The Chief Justice invoked the Constitution in the spirit in which the Founding Fathers would have written it. When they declared that a man's property was sacred, they would have included slaves within their definition of property.

To the Chief Justice, this meant that slavery was legal in every state of the Union.