Quartus II generates empty tdf file, then fails

So, your Qsys system is the only entity in your design and is your top level? What file(s) have you added to your Quartus project?

Qsys, by default, generates verilog source files, not AHDL. If you have captured your design entirely within Qsys then, having saved and generated your system, you need only add the <my_qsys_system>.qsys file to your Quartus project.

You mention the "db" folders - you shouldn't be digging around in there. These are temporary files that Quartus uses/creates. They never contain design source that you need to add to your Quartus project.

Cheers,

Alex