dbarron The case is Emperor has W10 installed in MBR. Solus is installed in EFI.
Emperor It's normal that it doesn't display the password (not even asterisks) for security reasons. No output means usually that the command is successful.
Emperor Solus uses GRUB only in MBR mode. So either you have to boot into Windows via your motherboard's boot menu (which works as intended) or reformat your Solus HDD and reinstall in MBR mode.
In my honest opinion the best option would be to reinstall W10 in EFI, and select the OS via motherboard's boot menu, as that's the intended way.