I bought a used evga 1080 graphic card. The proprietary power supply is rated 300w.1080? do you mean 3080?
why 24 to 12pin?
which power supply are you using? Better get an appropriate PSU than adapters which can burn down your house 😉
I think found one-so you will need a new PSU with a ATX main to acer motherboard adapter
you can find such on e.g. . amazon.com