Does anyone know how the wraith spawns? I've played about a dozen games since leviathans and still haven't seen one
To add to what Blorg Federation Member said.
1.) Every Leviathan has a % chance to appear in a galaxy which grows the larger a galaxy is.
I think for the maximum galaxy size in the console version it's around 60%.
2.) The spectral wraith is initially hidden in a random pulsar. Any empire that scans this pulsar (even the AI) will trigger the countdown for the wraith.
3.) Once the countdown is triggered, it will spawn once that hits 0. Since the wraith is one of the few mobile leviathans there is a chance it might wander into your empire even though you never triggered it, but it might also wander into a fallen empire and get killed.