Question about Sapphire Radeon RX 580 Pulse

Yes, you need a special cable to put this card in a Mac Pro 5.1 and power it properly.
To my knowledge, this kind of connector is not available from Digitec.
You need a Dual Mini 6 Pin to 8 Pin PCI-e. The 2 Minis are attached to the Mac Pro motherboard and the 8 Pin to the video card. There is a cable available from Digitec that looks similar but is not equipped with a mini 6 pin but a classic 6 pin and therefore, it won't work! It is not possible to find this cable on the market. For my part, cable ordered online from a supplier in Hong Kong
Another "off topic" point. This card is not equipped with a Mac compatible EFI, so black screen until the machine is fully booted and memory, only usable from High Sierra.