12 volt 8000 watt battery discharge protection
8000W / 12V = ~666A, which is not going to be trivial to switch.  Most power devices are designed to be run at higher voltages, with lower current to avoid this exact situation. 

That being said, most industrial size relays are called contactors. 
If you can 


100% guarantee that the contactor will never be switched while under load, and
assuming you are not trying to meet building codes, 


you could use a three phase contactor with all phases connected in parallel to switch the DC Positive line. 

The better overall approach in my opinion would be to use an inverter with a low voltage cutoff!