You are again confusing heat treatment's HARDENING phase with tempering.
Hardening is done from the critical temperature where the steel is no longer magnetic, it varies on the steel composition a bit, 800 is just an approximation.
Tempering is indeed done at a much lower temperature in the range you mentioned to relieve brittleness and give a softer but tougher structure as a final phase.
Using the term 'Tempering' to cover both phases is inaccurate. Differential heat treating where a blade is hardened at the edge and self tempered at the spine is a technique used by some experienced bladesmiths mostly with varying results.
The live slave thing is of course a myth, but corpses would be readily available.
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