Show me one law that legally describes the condition of slavery?
It can't be done. There was no legal definition of slavery in the United States government until after the Civil war.
That point is moot, but Article IV, Section 2 should satisfy your desire; the Thirteenth Amendment was enacted to BAN any property ownership of slaves. As was referenced upthread by Non-Sequitur, I would also refer to Scott vs. Sanford.
Slaves were deemed "property"; "CHATTEL", as in "chattel slavery" has its root from "CATTLE". Owning cattle or livestock wasn't mentioned in the Constitution as a specific legal right either - neither are the conditions describing livestock ownership defined, but domesticated animals were considered a protected "property" of their "rightful" owners.
The SCOTUS of that day believed property ownership of slaves was a legitimate Constitutional right and ruled thusly to protect it.